Cationic Copolymers of Norbornylized Seed Oils for Fiber-Reinforced Composite Applications

Abstract

Norbornylized seed oils, i.e., norbornylized linseed oil (NLO), norbornylized soybean oil (NSO), and norbornylized high oleic soybean oil (NHOSO), were synthesized via the Diels-Alder reaction of seed oil and dicyclopentadiene (DCPD) at high temperature (∼235 °C) and high pressure (∼80 psi), followed by cationic copolymerization using DCPD with boron trifluoride diethyl etherate catalyst. Norbornylized seed oils were characterized using H1 nuclear magnetic resonance (NMR), attenuated total reflectance-Fourier transform infrared, and gel permeation chromatography (GPC). Copolymers were formulated with four different DCPD contents, and curing was investigated using dynamic differential scanning calorimetry (DSC) measurements. It was found that the curing followed NLO > NSO > NHOSO with NLO having the highest exotherm, lowest activation energy, and lowest onset temperature. Furthermore, the gelation times were the least for NLO-DCPD copolymers. As anticipated, the degree of unsaturation and norbornene moieties strongly influenced the curing of copolymer thermosets. The copolymer products were compression-molded into thermosets and characterized by DSC, Soxhlet extraction, thermogravimetric analysis (TGA), H1 NMR, solid-state C13 NMR, and GPC. NLO-DCPD thermosets demonstrated high cure, higher thermal stability, glass transition temperature, and cross-linking capability compared to the other seed oil-DCPD counterparts. NMR and GPC results further suggested that bis-allylic and norbornene units concomitantly participated very actively during the cationic curing reaction. Moreover, scanning electron microscopy images of glass fiber-reinforced NLO-DCPD copolymer composites demonstrated good interfacial adhesion between the polymer matrix and fiber phases, imparting enhanced thermo-mechanical properties. This research opens a new venue for higher biobased greener polymer constituent for composite applications.

Figure 1

Structure of (a) dicyclopentadiene, (b) boron trifluoride diethyl etherate catalyst represent triglyceride unit fatty acid structures of (c) linseed oil, (d) soybean oil, and (e) high oleic soybean oil.

Figure 2

(a) Schematics of the HEL reactor and (b) reactor pressure as a function of reactor temperature: NLO batches.

Scheme 1. Reaction Pathway of Norbornylization of Linseed, Soybean, and High Oleic Soybean Oil and Summary of Its Physical Characteristics

Figure 3

DSC cure study of NLO, NSO, and NHOSO. (a) Onset temperature, (b) peak temperature, (c) enthalpy, and (d) activation energy as a function of DCPD loading.

Figure 4

H1 NMR spectra of the NLO-DCPD copolymer soluble extracts after Soxhlet extractions.

Figure 5

Solid-state C13 NMR spectra of NLO-DCPD copolymer insoluble materials after Soxhlet extraction.

Scheme 2. Cationic Copolymerization of NLO and DCPD

Figure 6

DSC thermogram of norbornylized seed oil-DCPD thermosets showing the T_g (a) NHOSO-DCPD-20, (b) NHOSO-DCPD-40, (c) NHOSO-DCPD-60, (d) NSO-DCPD-20, (E) NSO-DCPD-40, (f) NSO-DCPD-60, (g) NLO-DCPD-20, (h) NLO-DCPD-40, and (i) NLO-DCPD-60.

Figure 7

SEM images of fractured surfaces of NLO-DCPD-20 composites at 250× (a) and (b) 500× magnification.

Figure 8

Storage modulus and loss factor (tan delta) for NLO-DCPD-20 composites.

Figure 9

TGA thermogram of LO, NLO, and NLO-DCPD copolymer thermosets and composites.

Scheme 3. Illustration of (a) Differences in the Norbornylized Seed Oil Chemical Structure and (b) Chemical Units Responsible for Concomitant Reactions During Cationic Curing in NLO-DCPD