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Research Papers

Interlaminar Toughening of GFRP—Part I: Bonding Improvement Through Diffusion and Precipitation

[+] Author and Article Information
Dakai Bian

Department of Mechanical Engineering,
Columbia University,
New York, NY 10027
e-mail: db2875@columbia.edu

Bradley R. Beeksma, Y. Lawrence Yao

Department of Mechanical Engineering,
Columbia University,
New York, NY 10027

D. J. Shim, Marshall Jones

GE Global Research,
Niskayuna, NY 12309

1Corresponding author.

Manuscript received November 21, 2016; final manuscript received February 14, 2017; published online March 24, 2017. Assoc. Editor: Donggang Yao.

J. Manuf. Sci. Eng 139(7), 071010 (Mar 24, 2017) (9 pages) Paper No: MANU-16-1609; doi: 10.1115/1.4036126 History: Received November 21, 2016; Revised February 14, 2017

A low concentrated polystyrene (PS) additive to epoxy is used, since it is able to reduce the curing reaction rate but not at the cost of increasing viscosity and decreasing glass transition temperature of the curing epoxy. The modified epoxy is cocured with a compatible thermoplastic interleaf during the vacuum assisted resin transfer molding (VARTM) to toughen the interlaminar of the composites. Using viscometry, the solubilities of thermoplastics (TPs) polycarbonate (PC), polyetherimide (PEI), and polysulfone (PSU) are determined to predict their compatibility with epoxy. The diffusion and precipitation process between the most compatible polymer PSU and epoxy formed semi-interpenetration networks (semi-IPN). To optimize bonding adhesion, these diffusion and precipitation regions were studied via optical microscopy under curing temperatures from 25 °C to 120 °C and PS additive concentrations to epoxy of 0–5%. Uniaxial tensile tests were performed to quantify the effects of diffusion and precipitation regions on composite delamination resistance and toughness. Crack paths were observed to characterize crack propagation and arrest mechanism. Fracture surfaces were examined by scanning electron microscopy (SEM) to characterize the toughening mechanism of the thermoplastic interleaf reinforcements. The chemically etched interface between diffusion and precipitation regions showed semi-IPN morphology at different curing temperatures. Results revealed deeper diffusion and precipitation regions increase energy required to break semi-IPN for crack propagation resulting in crack arrests and improved toughness.

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Figures

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Fig. 1

Phase diagram of nucleation growth (NG) and spinodal decomposition (SD). The system can lower its free energy by separating into two phases with an interphase between c and c+. Within the range of c/cS− and c+/cS+, phase separation is due to nucleation and growth. Within the range of cS− and cS+, phase separation is due to spinodal decomposition.

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Fig. 2

(a) Schematic of semi-interpenetration network. Semi-interpenetration network is formed by the entanglement between the long-chain thermoplastic molecules and the crosslinked thermosets. (b) Schematic of crack propagation in the diffusion and precipitation region. When the crack propagates, it needs to break the semi-interpenetration network structure.

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Fig. 3

Vacuum assisted resin transfer molding experiment setup. The entire process is under high vacuum, which provides the lowest porosity in the final specimen.

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Fig. 4

Experimentally determined intrinsic viscosity of PSU, PEI, and PC by using Eq. (3)

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Fig. 5

Average diffusion depths of the specimens (a) with 0% and 5% polystyrene modified epoxy cured from room temperature to 120 °C and (b) with 0–5% polystyrene modified epoxy cured at 80 °C and 120 °C

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Fig. 6

PSU thermoplastic diffusion and precipitation region with 5% PS modified epoxy cured at 120 °C is measured from optical microscopy imaging of the thermoplastic (TP)–thermoset (TS) interface. The thermoset diffusion into thermoplastic is characterized by a gradient island-shaped phases, and the thermoplastic precipitation region is characterized by the dispersed PSU in the epoxy after curing.

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Fig. 7

PSU thermoplastic diffusion and precipitation region with 5% PS modified epoxy cured at 80 °C is measured from optical microscopy imaging of the thermoplastic (TP)–thermoset (TS) interface

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Fig. 8

PSU thermoplastic diffusion and precipitation region with nonmodified epoxy cured at 120 °C is measured from optical microscopy imaging of the thermoplastic (TP)–thermoset (TS) interface

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Fig. 10

EDX line scan across TS–TP interface of the specimen (a) under 5% PS modified epoxy cured at 80 °C and (b) under 5% PS modified epoxy cured at 120 °C. The dashed lines indicate the diffusion and precipitation region.

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Fig. 9

Average precipitation depths of the specimens (a) with 0% and 5% polystyrene modified epoxy cured from room temperature to 120 °C and (b) with 0–5% polystyrene modified epoxy cured at 80 °C and 120 °C

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Fig. 11

PSU thermoplastic diffusion and precipitation region in the fiber matrix with 5% PS modified epoxy curing at 120 °C. The dashed line represents the boundary of TS–TP interface. The arrows represent the locations where there are precipitates.

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Fig. 12

PSU thermoplastic diffusion and precipitation region in the fiber matrix with 5% PS modified epoxy curing at 80 °C. The dashed line represents the boundary of TS–TP interface. The arrows represent the locations where there are precipitates.

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Fig. 13

PSU thermoplastic diffusion and precipitation region in the fiber matrix with nonmodified epoxy curing at 120 °C. The dashed line represents the boundary of TS–TP interface.

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Fig. 14

Representative strain–stress curves obtained from the strain gauge mounted on the drop-off layer in uniaxial tensile tests: reference specimen (without interleaf) and interleaved specimens with 5% PS modified epoxy cured at 80 °C and 120 °C

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Fig. 15

Toughness of PSU interleaved specimens from 0.5% to 5% PS modified epoxy cured at 80 °C and 120 °C. The error bars represent standard errors.

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Fig. 16

SEM image of the fracture surface of (a) the reference specimen without interleaf, (b) the interleaved specimen with diffusion and precipitation, and (c) high resolution close up of the fracture surface

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