Research Papers

Interlaminar Toughening of Fiber-Reinforced Polymers by Synergistic Modification of Resin and Fiber

[+] Author and Article Information
Dakai Bian

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

Jason C. Tsui

Department of Mechanical Engineering,
Columbia University,
New York, NY 10027
e-mail: jct2160@columbia.edu

Robert R. Kydd

Department of Mechanical Engineering,
Columbia University,
New York, NY 10027
e-mail: rrk2139@columbia.edu

D. J. Shim

GE Global Research,
Niskayuna, NY 12309
e-mail: shim.dj@gmail.com

Marshall Jones

GE Global Research,
Niskayuna, NY 12309
e-mail: jonesmg@ge.com

Y. Lawrence Yao

Department of Mechanical Engineering,
Columbia University,
New York, NY 10027
e-mail: yly1@columbia.edu

1Corresponding author.

Manuscript received November 29, 2017; final manuscript received October 30, 2018; published online June 13, 2019. Assoc. Editor: Y. B. Guo.

J. Manuf. Sci. Eng 141(8), 081008 (Jun 13, 2019) (12 pages) Paper No: MANU-17-1742; doi: 10.1115/1.4043836 History: Received November 29, 2017; Accepted November 02, 2018

The synergistic effect of combining different modification methods was investigated in this study to improve the interlaminar toughness and delamination resistance of fiber reinforced polymers (FRP). Epoxy-compatible polysulfone (PSU) was end-capped with epoxide group through functionalization, and the fiber surface was chemically grafted with an amino functional group to form a micron-size rough surface. Consequently, the long chain of PSU entangles into cross-linked thermoset epoxy network, additionally, epoxide group on PSU further improves the bonding through chemical connection to the epoxy network and amino group on the fiber surface. The combined modification methods can generate both strong physical and chemical bonding. The feasibility of using this method in vacuum-assisted resin transfer molding was determined by rheometer. The impact of formed chemical bonds on the cross-linking density was examined through glass transition temperatures. The chemical modifications were characterized by Raman spectroscopy to determine the chemical structures. Synergistic effect of the modification was established by mode I and mode II fracture tests, which quantify the improvement on composites delamination resistance and toughness. The mechanism of synergy was explained based on the fracture mode and interaction between the modification methods. Finally, numerical simulation was used to compare samples with and without modifications. The experiment results showed that synergy is achieved at low concentration of modified PSU because the formed chemical bonds compensate the effect of low cross-linking density and interact with the modified fiber.

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Fig. 1

Chemical reactions in epoxy curing [17] and modification process [10,13]: (a) primary amine from hardener has open-ring reaction with epoxide group from epoxy and generates secondary amine, (b) secondary amine reacts with the epoxide group, (c) etherification reaction, (d) amino group grafting onto glass fiber surface, and (e) epoxide group end-capped PSU

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Fig. 2

Bonding in the cured epoxy matrix. The physical bonding here is due to the entanglement of long-chain thermoplastics with the cross-linked thermosets epoxy, which is known as semi-interpenetration network. The chemical bonding due to the modifications are among glass fiber surface to PSU, glass fiber surface to curing epoxy, and PSU to curing epoxy.

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Fig. 3

Schematic of crack propagation between glass fiber surface and cured epoxy under different conditions. (a) With no modifications, the crack lies on the interface between nontreated glass fiber surface and epoxy. Weak intermolecular forces are the only major factors holding two materials together. (b) Under mode I fracture, the crack propagates through the interface between chemical treated glass fiber and modified epoxy. The surface of treated glass fiber becomes rough. The increased contact area not only improves the adhesion strength but also generates micromechanical interlocks. (c) Under mode II fracture, the crack propagates through the interlaminar region with crack bridging phenomenon.

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Fig. 4

(a) Viscosity for epoxy with various concentrations of modified PSU from room temperature to 120 °C and (b) curing kinetics of neat epoxy under 80 and 120 °C

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Fig. 5

Raman spectroscopy for (a) PSU and (b) epoxide end-capped PSU. The new peak appeared ∼1240 cm−1 was the evidence of the epoxide ring grafting onto the polymer chain.

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Fig. 6

Raman spectroscopy for (a) nonmodified glass fiber surface and (b) chemically treated glass fiber surface. The peak appeared ∼995 cm−1 is the evidence of the amino functionality group.

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Fig. 7

SEM images of glass fiber surface morphology (a) before chemical treatment, (b) after chemical treatment, and (c) after chemical treatment at high magnification

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Fig. 8

Glass transition temperatures of cured epoxy with different concentrations of modified polysulfone. Error bars represent the standard errors. The advantage of bonding between additives and epoxy can compensate for the influence of additive at low additive concentration, leading to increased glass transition temperature.

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Fig. 9

Optical microscopy of phase morphology in the cured epoxy etched by methyl chloride to remove the PSU-rich region (a) with 0.5 wt% PSU and (b) with 5 wt% PSU. The holes on the surface were due to the removal of PSU, which were highlighted with arrows.

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Fig. 10

Mode I critical energy release rate of the specimen with different concentrations of modified PSU. Error bars represent the standard errors.

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Fig. 11

Synergistic study of mode I fracture test. Specimens with individual modification method were compared with the specimens with combined modification methods. Slight synergistic effect from two individual modifications methods was found in mode I fracture test.

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Fig. 12

SEM images of mode I fracture surface morphology of (a) 2% PSU with nonmodified glass fiber, (b) 0% PSU with modified glass fiber, and (c) 2% PSU with modified glass fiber

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Fig. 13

Mode II critical energy release rate of the specimen with different concentrations of modified PSU. Error bars represent the standard errors.

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Fig. 14

Synergistic study of mode II fracture test. Specimens with individual modification methods were compared with the ones with combined modification methods. The further improvement of delamination in mode II was considered due to the more modified epoxy resin participating in the plastic deformation.

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Fig. 15

SEM images of mode II fracture surface morphology of (a) 0% PSU with modified glass fiber, (b) 2% PSU with nonmodified glass fiber, and (c) 2% PSU with modified glass fiber

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Fig. 16

Simulation of mode I fracture. Specimen was 160 mm in length and 4.15 mm in thickness. The left end was under a displacement loading. Initially the precrack was 60 mm in length and placed 5 µm above the middle of the interlaminar region. The contour map in magnified regions represented the status of crack. Value of 1 in that element represented total fracture and value of 0 in th at element represented zero fracture. The value in between indicated there still existed traction on the crack surface, which was considered as partial fracture. The crack growth matched the previous crack propagation analysis for mode I in Fig. 3(b).

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Fig. 17

Mode I simulation results versus experiment results

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Fig. 18

Simulation of mode II fracture. The shear deformation was represented by the misalignment of the cells along the predefined interlaminar region. The contour map in magnified region represented the status of crack.

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Fig. 19

Mode II simulation results versus experiments results. The overestimation of the simulation results after the crack initiation was mainly because the crack bridging phenomenon dissipated more energy than single crack growth modeled in the simulation.



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