Effect of Self-Healing by Dicyclopentadiene Microcapsules on Tensile and Fatigue Properties of Epoxy Composites

Abstract

Microcapsules of urea-formaldehyde (UF) containing dicyclopentadiene (DCPD) were synthesized by the in situ polymerization technique for self-healing of epoxy. The dispersion of microcapsules in the epoxy matrix was achieved using ultrasonication. Composites of epoxy, having 0.5, 1.0, 1.5, and 2.0 wt.% of microcapsules capable of self-healing, were prepared. The shape and size of the microcapsules were determined by field emission electron microscopy. Spherical capsules of DCPD, with an average diameter of 172 nm, were obtained. Investigation of tensile properties indicated a decrease in the tensile modulus with an increase in wt.% of microcapsules. There was a reduction of 22%, 27%, 39%, and 30% in the elastic modulus of composites for 0.5, 1.0, 1.5, and 2.0 wt.% of microcapsules, respectively. Tensile strength was found to increase with an increase in wt.% of microcapsules. The tensile strength of the composites increased by 33%, 20%, 8%, and 21% for 0.5, 1.0, 1.5, and 2.0 wt.% of microcapsules, respectively, in comparison with that of neat epoxy. The fatigue life of composites was investigated by conducting uniaxial tension-tension fatigue tests at constant stress amplitudes of 20, 25, 30, and 35 MPa, at a constant stress ratio (R = 0.1) and a frequency of 3 Hz. The fatigue life of composites increased with an increase in wt.% of microcapsules in comparison with that of neat epoxy. It was found that the fatigue life of the composites decreased with 1.5 and 2.0 wt.% of microcapsules in comparison with composites with 0.5 and 1.0 wt.% of microcapsules. The fracture surfaces of the tested samples were examined with the help of scanning electron microscopy (SEM) to understand the various mechanisms responsible for the change in modulus, strength, failure strain, and fatigue life of composites.

Keywords: fatigue life; mechanical characterization; polymer composites; self-healing composites; tensile properties.

Figure 1

Process of synthesis of microencapsulation of DCPD [3].

Figure 2

Mechanism of DCPD polymerization [23].

Figure 3

Fabrication process of composites.

Figure 4

Tensile and fatigue test specimen (dimensions in mm) [25].

Figure 5

FESEM images of DCPD microcapsules synthesized at different agitation rates.

Figure 6

Average size of DCPD microcapsules at 2000 rpm.

Figure 7

SEM image of composites showing dispersion for different wt. percentages.

Figure 8

(a) Stress–strain diagrams of composites for different wt.% of microcapsules. (b) Initial linear portion of the stress–strain curve for the composites in region A,B,C of (a). (A—Stress up to 20 MPa, B—Strain up to 0.009).

Figure 8

(a) Stress–strain diagrams of composites for different wt.% of microcapsules. (b) Initial linear portion of the stress–strain curve for the composites in region A,B,C of (a). (A—Stress up to 20 MPa, B—Strain up to 0.009).

Figure 9

(a) Elastic modulus vs. wt.% of capsules, (b) tensile strength vs. wt.% of capsules, and (c) failure strain vs. wt.% of capsules.

Figure 10

S-N curves of neat epoxy and epoxy–DCPD composites.

Figure 11

Percentage increase in number of cycles to failure (Nf) for epoxy/DCPD composites.

Figure 12

Fractured surfaces of neat specimen at (a) 20 MPa, (b) 25 MPa, (c) 30 MPa, and (d) 35 MPa.

Figure 13

Fractured surfaces of composite having 1.0 wt.% capsules tested at (a) 35 MPa, (b) 30 MPa, (c) 25 MPa, and (d) 20 MPa.

Figure 14

SEM image of fractured surface at 25 MPa.

Figure 15

Fractured surface of samples tested at 35 MPa: (a) neat epoxy and composites, (b) 0.5 wt.%, (c) 1.0 wt.%, and (d) 1.5 wt.%.