Processing, structure, and properties of fibers from polyester/carbon nanofiber composites

Abstract

Poly(ethylene terephthalate) (PET) resin has been compounded with carbon nanofibers. The amount of carbon nanofibers utilized in each case was 5 wt.%. Compounding methods included ball-milling, high shear mixing in the melt, as well as extrusion using a twin-screw extruder. PET/CNF composite resins were melt-spun into fibers using the conventional PET fiber spinning conditions. Morphology and mechanical properties of these composite fibers have been studied. The results show that CNFs can be incorporated into PET matrix with good dispersion. Compressive strength and torsional moduli of PET/CNF composite fibers were considerably higher than that for the control PET fiber.

Introduction

Single [1], [2], [3] and multiwall carbon nanotubes [4], [5], [6], [7], [8] as well as vapor-grown carbon nanofibers [9] are promising candidates for reinforcing various polymer matrices. Vapor grown carbon nanofibers which typically have diameters in the 50–200 nm range are also referred to as multiwall carbon nanotubes. By comparison, the diameters of single wall carbon nanotubes (SWNTs) are of the order of 1 nm and a multiwall carbon nanotube (MWNT) diameter can be upwards of a few nanometers. Carbon nanotubes as well as nanofibers exhibit good thermal [10] and electrical conductivity [11] and possess excellent mechanical properties [12]. Thermoplastics such as polypropylene [13], [14], [15], [16], [17], [18], polycarbonate [19], [20], [21], [22], [23], nylon [15], [24], poly(ether sulfone) [25], poly (phenylene sulfide) [26], acrylonitrile-butadiene-styrene [25], thermosets such as epoxy [26], [27] as well as thermoplastic elastomers such as butadiene-styrene diblock copolymer [28] have been reinforced with carbon nanofibers (CNFs). Carbon nanofibers have been blended into polymer matrices using conventional mixing methods such as use of twin-screw extruder [13], [20], [21], [22], high shear mixer [16], [24], as well as two-roll mill [26]. To improve CNF dispersion and nanofiber/polymer interfacial strength, carbon nanofibers have been purified [16], ball-milled [15], [26], functionalized [16], and surface treated with plasma [15], [26].

Ball-milled CNF/nylon [15] composites have slightly improved tensile strength and double the modulus of unreinforced material, while ball milled CNF/PP [15] composites have double the tensile strength and quadruple the modulus of unreinforced material. CNF/PP composites made by air-etched fiber, CO₂-etched fibers, and fibers covered with low concentrations of aromatics possessed significantly better mechanical properties than did fibers whose surface was heavily coated with aromatics [15]. In studying ball milled and plasma treated carbon nanofibers in epoxy and poly(phenylene sulfide) (PPS) matrices, Patton et al. [26] found flexural strength to increase by up to 68% over neat resin at 19.2% reinforcement.

Along with the efforts towards improvement of matrix polymer mechanical properties, research on the preparation of multifunctional materials has also been carried out [14], [17], [26], [29], [30], [31]. Lozano et al. [17] prepared CNF/PP composites and observed a percolation threshold for electrical conduction of 9–18 wt.% CNF. With addition of 1 and 5 wt.% HDPE, Wu et al. [31] found the percolation threshold of PMMA/CNF composites was reduced from 8.0 to 4.0 phr (parts per hundred parts) and that it could be further reduced to 1.5 phr after annealing. The drastic decrease in percolation threshold was attributed to the selective adsorption of HDPE in PMMA/CNF composites.

The rheological [17], [22], [23], crystallization [16], and thermal degradation behavior [16] of polymer/CNF composites have also been studied. Lozano et al. showed that the incorporation of 30 wt.% CNF into PP raised the working temperature of the resin by 100 °C [16]. They also showed that addition of CNF increased the rate of PP crystallization [16]. Increased polypropylene crystallization rates have also been reported with addition of single wall carbon nanotubes (SWNT) [32].

In this study, different grades of CNFs have been melt blended into poly(ethylene terephthalate). Melt blended PET/CNF composites have been spun into fibers. Processing, properties, and morphology results of these studies are reported herein.