A new self-healing epoxy with tungsten (VI) chloride catalyst

Abstract

Using self-healing materials in commercial applications requires healing chemistry that is cost-effective, widely available and tolerant of moderate temperature excursions. We investigate the use of tungsten (VI) chloride as a catalyst precursor for the ring-opening metathesis polymerization of exo-dicyclopentadiene (exo-DCPD) in self-healing applications as a means to achieve these goals. The environmental stability of WCl6 using three different delivery methods was evaluated and the associated healing performance was assessed following fracture toughness recovery protocols. Both as-received and recrystallized forms of the WCl6 resulted in nearly complete fracture recovery in self-activated tests, where healing agent is manually injected into the crack plane, at 12wt% WCl6 loading. In situ healing using 15wt% microcapsules of the exo-DCPD produced healing efficiencies of approximately 20%.

Figure 1

SEM images of WCl₆: (a) as-received, (b) recrystallized and (c) wax protected. (Scale bars, 100 μm.)

Figure 2

Localized TDCB specimen geometry (all dimensions in millimetres; Rule et al. 2007).

Figure 3

Mechanical assessment of self-activated specimens. (a) Representative load versus displacement data for recrystallized specimens (arrow indicates the peak of the healed response). (b) Image of poly(DCPD) fibrils spanning the fracture plane (scale bar, 1 mm). (c) Representative load versus displacement data for as-received specimens. (d) Representative load versus displacement data for wax-protected samples.

Figure 4

UV-vis spectra results for (a) as-received with inset of spectra from 300 to 400 nm, (b) recrystallized and (c) wax-protected WCl₆ upon exposure to air at 22°C and 30% relative humidity for 0, 2 and 24 h. The characteristic peaks for WCl₆ are located at 328 and 372 nm and for WOCl₄ at 355 nm (Thorn-Csányi & Timm 1985).

Figure 5

SEM images of epoxy fracture surfaces for samples containing as-received WCl₆ (12 wt%). (a) Significant interfacial debonding is revealed when no coupling agent is used. (b) The addition of 1 wt% silane coupling agent (3-glycidoxypropyltrimethoxysilane) dramatically reduces the amount of interfacial debonding (scale bars, 50 μm).

Figure 6

Self-activated mechanical response of a sample containing 12 wt% recrystallized WCl₆ with 1 wt% coupling agent in the matrix.

Figure 7

SEM images of epoxy fracture surfaces for samples containing as-received WCl₆ (12 wt%). (a) Significant clustering of tungsten particles in the absence of mechanical mixing. (b) Particle dispersion achieved by mechanical mixing at 500 r.p.m. (scale bars, 500 μm).

Figure 8

Self-activated mechanical response of a sample containing 12 wt% as-received WCl₆ dispersed with mechanical mixing at 500 r.p.m.

Figure 9

(a) In situ mechanical response of a sample containing 12 wt% recrystallized WCl₆ and 15 wt% 188±31 μm capsules and (b) SEM image of the fracture surface of an in situ sample after healing and refracture (scale bar, 500 μm).