Joseph M. Deitzel

Compressive Strength Analysis for High Performance Fibers with Different Modulus in Tension and Compression

The assumption of equal tensile and compressive modulus necessary to determine single fiber axial compressive strength from the elastica loop test is relaxed by deriving a compressive strength equation based on the analysis of the flexural response of a fiber with different modulus in tension and compression. Previously determined tensile (E1t) and compressive (E1c ) modulus values for different high performance organic fibers with varying degrees of lateral molecular interactions are used to determine fiber compressive strength. The importance of using the bi-moduli equation becomes evident in the case of fibers that lack strong intermolecular interactions, since calculations done with the original loop test equation can result in an overestimation of the compressive strength on the order of 80%, as seen for the poly-(p-phenylene benzobisoxazole) fiber PBO. Kink band angles measured from looped fiber specimens are documented and correlated to the calculated compressive strength values.

Effect of Fiber Surface Texture Created from Silane Blends on the Strength and Energy Absorption of the Glass Fiber/Epoxy Interphase:

Most of the research to date has focused on tailoring the interphase adhesion by controlling the degree of chemical bonding between fiber and resin. The interfacial shear strength (IFSS) has been increased as much as 40% by modified chemical surface bonding [1—3]. However, it is well known that increasing the interfacial strength of the fiber reinforced polymeric composite material often leads to a reduction in the fracture toughness and vice versa [4—12]. In this study, the effects of mechanical interlocking, in addition to chemical bonding on the strength and energy absorption of glass fiber/epoxy interphase, were studied by creating texture on the fiber surface through the phase separation of silane blends. A series of tetraethoxysilane (TEOS)/3-glycidoxypropyltrimethoxysilane (GPS) blends in solutions of ethanol and water was selected to treat the glass fiber surface. The fiber coated with different surface treatments shows the change in fiber surface morphology due to the addition of TEOS. X-ray photoelectron spectroscopy (XPS) analysis showed that the GPS preferentially migrates to the coating surface which suggests that phase separation induced by the silane blend was the primary mechanism for the texture formation. Atomic force microscopy (AFM) was used to scan the fiber surface after the coating and the fiber surface texture was quantified by the roughness values. In addition, a single-fiber Microdroplet shear test was conducted to assess the interfacial properties between the textured glass surface and an epoxy matrix. Traditionally, interfacial shear strength is the only quantity that was determined from the load vs. displacement curve after microdroplet test. In this study, a new data-reduction scheme was developed to determine the energy absorption due to different failure mechanisms by taking into consideration both machine compliance and fiber stretching in the energy calculation. The results show as much as a three-fold increase in specific sliding energy absorption without sacrificing interfacial shear strength. The examination of failure surfaces shows that failure mode propagates through the textured interphase in a more tortuous path, which results in greater degree of energy absorption during fiber—matrix pullout. This study shows the potential for using chemical bonding and mechanical interlocking effects to improve both strength and energy absorption in fiber reinforced composites.

The effects of environmental conditioning on tensile properties of high performance aramid fibers at near-ambient temperatures

Aramid and aramid copolymer fibers are used in a wide variety of military and civilian applications; however, the long-term effects of environmental exposure on tensile properties are still not well understood. The current effort investigates the effect of hygrothermal conditioning on the tensile properties of Kevlar® KM2 ®, Twaron®, and the newly available Russian copolymer, Armos® high performance fibers. Moisture uptake studies show that at room temperature, water diffuses more slowly into the copolymer Armos ® (D = 8.7 × 10-13 cm2/s) compared to the Kevlar® KM2® and Twaron® homopolymers (D = 2.16 × 10-12 cm2/s and D = 1.8 × 10 -12 cm2/s, respectively). Tensile properties have been measured for these aramid fibers that have been conditioned in water at 40°C, 60°C, 80°C, and 100°C for periods of 17 and 34 days. For both aramid and aramid copolymer fibers, hygrothermal conditioning did not significantly change fiber tensile properties below 80°C. At the most extreme condition of 100°C, 34 days, aramid fibers showed significant loss of tensile strength (58% for KM2 and 34% for Twaron®), while a reduction in tensile strength of 13% (Armos®) was observed for aramid copolymer (Armos®) fibers. Conditioned fibers exhibited no significant change in mass as a result of the conditioning procedure and FTIR spectroscopy results did not indicate signs of chemical or thermo-oxidative change due to hygrothermal conditioning. These results suggest that in aramid fibers, the primary mechanism of degradation at temperatures between 80°C and 100 °C is due to the ingress and egress of moisture in the highly ordered core structure of the fiber. The presence of water in the intercrystalline regions of the fiber core enable segmental chain motion that can relax tie molecules, alter crystal orientation, and change apparent crystallite size. Because of differences in molecular architecture and phase morphology, the aramid copolymer, Armos®, is less susceptible to degradation of tensile properties under these conditions.

Drawing of Spatially Oriented Electrospun Fibers

A simple approach to electrospinning has been developed that enables the collection of polymer, ceramic, and multiphase composite fibers, in quantity, with a high degree of spatial orientation. It has been demonstrated that a careful choice of solvent effectively eliminates the onset of the characteristic “bending” instability that is commonly associated with the electrospinning process. This allows collection of spatially oriented submicron electrospun fibers on a rotating drum without the need for elaborate mechanical or electrostatic manipulation of the electrospinning jet and/or collection target. Fibers have been electrospun from a series of model polyethylene oxide/CHCL3 solutions with a range of conductivities. The experimental data confirms theoretical predictions that the onset of the bending instability is a function of the available “free” charge in the solution, which in turn is strongly influenced by the dielectric constant of the solvent. The results show that fiber orientation becomes random as the conductivity increases, indicating the need for the surface charge density to exceed a critical threshold in order for the bending instability to initiate. Furthermore, it has been demonstrated that fiber diameter can be effectively controlled by controlling drum take-up speed. This method has been experimentally demonstrated with other low dielectric constant solvents and other common polymer, ceramic, composite materials.Copyright

High performance polyethylene fibers

Abstract High performance PE fibers are found in a wide variety of commercial and defense applications today. They represent the highest tensile strength to mass ratio of any known fiber material. The evolution of this class of fibers from a scientific curiosity in the first half of the 20 th century to the high performance ballistic grade materials in use today represents a significant portion of the history of polymer science as a whole. Each stage in the development of PE fiber processing, from Melt spinning to Solution spinning to finally Gel-spinning has provided key insights into fundamental behavior of linear, flexible polymer molecules in the melt state and in solution. Current research in employing sophisticated scanning probe microscopy and spectroscopic techniques are being used to map the interior structure of single PE filaments in great detail, providing insight into how processing history influences the sub-filament structure and the manner in which load is translated through the fiber. What follows is a brief history of this evolution of PE fibers and a discussion of the current areas of relevant research. A key challenge for investigators new to this topic is sorting through the vast amount of published work generated over the last 50+years. It is hoped that the information in the following chapter will serve as a starting point and provide both historical context and a solid foundation from which new threads of research into the processing and properties of high performance PE fibers can be initiated.

Studies of the Fiber-Matrix Interphase for Controlled Energy Absorption and Strength

Hybrid polysiloxane-based sizings with embedded silica and silica/latex nanoparticles were studied to improve Interfacial Shear Strength (IFSS) and energy absorption using the micro-droplet test method. The sizing mixture comprised of mixed polysiloxane with silica nanoparticles has been screened by varying one parameter at a time. The best mechanical performance was observed in the following concentration range: 1.0–2.0 wt% tetraethoxysilane (TES)/3-glycidoxypropyltrimethoxy silane (GPS) mixture, and 1.0–3.0 wt% Ludox® silica nanoparticles; 1.0–1.5 TES/GPS ratio, and 20 nm silica nanoparticles size. Increase of nanoparticles size leads to a decline in both strength and energy absorption, whereas chemical modification of nanoparticles surface has no significant effect compared to the baseline. The study also shows that the combination of hard (silica) and soft (latex) nanoparticles works best to maximize the strength and energy absorption. Hard particles increase surface roughness that increases interfacial strength and energy absorption during debonding whereas rubber particles contribute to improved energy absorption. Possible rational for such sizing composition/material property relationship will be discussed.Copyright