Influence of Processing Parameters on the Strength of Fluoride Glass Fibers

The different factors affecting the tensile strength of heavy-metal fluoride glasses are presented. Surface crystals were found to be responsible for the lower fiber strength. The tensile strength of fluoride glass fibers can be increased by controlling the preform surface and drawing in a dry atmosphere. Using lower draw temperature and dual polymeric coatings increases the strength. Finally, phosphate glass overclad on fluoride glass fiber can be used to prevent surface hydrolysis during the drawing process and increases the tensile strength of fibers due to formation of compressive stress on the outside of the glass fiber.     

Scanning Tunneling Microscopy of Optical Fiber Corrosion: Surface Roughness Contribution to Zero-Stress Aging

Strength, fatigue resistance, and zero-stress aging behavior control the long-term mechanical reliability of optical fibers. Zero-stress aging refers to the loss of strength of high-strength glass fibers after exposure to some corrosive environments in the absence of stress. Understanding the effect of the chemical environment under zero stress on the subsequent fracture strength of optical fibers is important because optical fibers in service will probably encounter water and other chemical species while exposed to zero- or low-stress conditions. In this work, the strength of fibers aged under zero-stress conditions at 80°C in deionized water has been measured. Scanning tunneling microscopy was also used to measure the roughening of the fibers from corrosion at intervals during the aging. The product of the median inert strength of fibers aged for various times and the square root of the roughness depth of fibers was constant within experimental error. The results show that surface roughening contributes to zero-stress aging in silica fibers.     

Zero stress aging in notched multi-component glass fibers

Degradation of glass under zero applied load in the presence of humidity at ambient temperature is of great interest to the container and fiber glass industries. The phenomenon is well documented for fused silica used in optical fibers, but has not been studied in detail for multi-component glasses. In this work notches of varying length (500-1500 nm) were placed with a focused ion beam into two types of multi-component glass fibers, E-glass (48 μm diameter) and soda-lime-silicate (35 μm diameter). Notched specimens were exposed to dry and humid conditions for up to 32 days. Transmission electron microscopy revealed the presence of extensive reaction products within the root of the notch, even after only 1 day of aging in the nominally dry environment for the soda-lime-silicate glass. Surprisingly, the extensive reactions have no measurable effect on the fiber strength. The uniaxial tensile strength of the notched glass fibers, measured with the fracture surface mirror radius method, does not follow a classic fracture mechanics prediction, implying that the notches are not classic Griffith flaws. Fracture mechanics is applied to show that sharpness at the notch base may be important, especially when subcritical crack growth is present during the strength measurement.     

The adhesion of elastomer-coated glass fibers to an epoxy matrix. Part I: The effect of the surface treatments on the tensile strength of the glass fibers

E-glass fibers were subjected to various surface treatments to study the interfacial adhesion with an epoxy matrix by means of the fragmentation test. The glass fibers considered were both untreated and treated with γ-aminopropyltriethoxysilane (γ-APS). In addition, glass fibers were coated with a thin layer of a crosslinkable elastomer including (or not) a silane coupling agent. To evaluate the effect of the coating process, the glass fibers were also passed through the pure solvent (washed fibers). The tensile strength of glass fibers at short length (near the critical length, l_c, for the fragmentation test) cannot be measured directly, thus, extrapolation techniques were used. The Weibull statistics technique was applied and accurately described the tensile strength data of the tensile strength at various fiber gauge lengths for the different surface treatments. Nevertheless, the Weibull parameter, m, changes with the gauge length, and therefore, the extrapolation value at l_c depends on the method. The tensile strength of the silane-treated glass fibers is higher than for the untreated fibers, in agreement with reported data. This effect could be attributed to the protection against water provided by the silane sizing. The coating process does not induce any damage of the glass fibers since the washed fibers display mechanical properties close to those obtained for the untreated fibers. An additional effect is observed for the elastomer-coated fibers which present the highest tensile strength. This effect could be attributed to an improved protection and/or elimination of the weakest filaments in the glass strand during the treatment.     

Effect of Water Vapor on the Sintering of Glass Powder Compacts

Shrinkage isotherms of powder compacts of two soda-lime glasses were measured in various partial pressures of water vapor. The rate of shrinkage increased with increasing partial pressure of water vapor. Shrinkage isotherms of porous glass of high silica content were also sensitive to the partial pressure of water vapor in the ambient atmosphere. Because the rate of shrinkage is proportional to surface tension and inversely proportional to viscosity, it is not possible to clearly delineate the effect of water on viscosity, but arguments are presented for a dependence of viscosity on the square root of water vapor partial pressure.     

Calculation and Interpretation of Surface Free Energy of Wetting of E-Glass by Vapors

The surface free energy of wetting of E-glass by water and benzene vapors was calculated from Bangham and Razouk's free-energy equation. The method of Fu and Bartell for determining specific surface area was used to analyze the microstructure. The surface free energies, which were computed from adsorption isotherms obtained at temperatures from 18° to 20°C, had values from -235 to -254 ergs/cm² and -71.1 to -72.4 ergs/cm² for water and benzene adsorption, respectively, depending on temperature, surface geometry, and sample treatment. The lower affinity of E-glass for benzene, which resulted in a smaller work of adhesion, suggests that active adhesion of hydrocarbons is impossible in the presence of bulk water. Microstructural analysis indicated that the water-modified E-glass surface contains micropores 23 Å in diameter.     

Relation Between Melt Treatment and Glass Fiber Strength

The scatter usually seen in strength data for virgin E-glass fibers can be reduced to a very low level by controlling the thermal history of the glass melt from which the fibers are formed. Provided the glass melt is heated substantially above the drawing temperature, strengths of subsequently produced groups of 10 specimens will have a coefficient of variation of ∽1%. Unless glass is prevented from stagnating in the nozzle of the fiber-forming apparatus during a pause in the drawing operation, a secondary effect, associated with the spontaneous creation of defects in the glass in the nozzle, causes fibers produced immediately after such a pause to exhibit low strengths and high scatter.     

Model for Strength of Brittle Materials Built up of Particles Joined at Points of Contact

Theoretical analysis of the strength of materials consisting of particles joined along contact areas small in relation to particle size resulted in the model where p denotes strength, D particle size, γ surface energy, d linear extent of the contact area, E modulus of elasticity for the particles, and θ porosity. The first factor has the same form as the Griffith model except that particle size is substituted for crack size; it shows that strength increases inversely with the square root of particle size. The model can be used as a guide in the design of materials such as mortars with low cement content, sand-lime bricks, soil stabilizers, and sintered products. The strengths of mortars made from finely ground quartz and cement and cured at high temperatures were 4 to 40 times higher than those of normal mortars with the same cement/water ratio.     

Adsorption of γ -UPS silane onto E-glass fibers from aqueous solutions

The deposition of γ-ureidopropyltrimethoxysilane (γ-UPS) from aqueous solutions onto plasma-treated E-glass fibers was investigated over a wide range of concentrations and pH values. At low concentrations for all the deposition pHs chosen, the silane was shown to form a surface consisting of silanols. At high concentrations, the silane was shown to form a different surface, consisting of condensed Si–O–Si groups. The ureidosilane thus produced a hydrophobic surface at high concentrations. Osterholtz and Pohl demonstrated that condensation of γ-APTES (γ-aminopropyltriethoxysilane) increased rapidly between pH 6 and 7, and that the hydrolysis was slow. This pH range corresponds to the most extensive adsorption of γ-UPS onto the E-glass surface, as judged from water contact angle, mass loss, and SEM results. The silane was deposited as patches onto the E-glass fibers. At the higher solution concentrations, an outer surface or 'skin' was formed over these patches, giving the appearance of completely siloxane polymer-coated fibers. The structure under the outer surface was open and porous.     

Evaluation of surface treatments for glass fibers in composite materials

The thermomechanical stability of a number of organosilane surface treatments for glass fibers was evaluated for use in a fiber reinforced epoxy resin. All of the silane coatings were found to improve the tensile strength of E-glass filaments, particularly at large gauge lengths. A phenylamino silane and an amino silane were particularly effective in this regard. The fiber/matrix interface was evaluated as a function of temperature and after exposure to boiling water using a single-fiber composite test. All silane coatings transmitted a higher interfacial shear stress than obtained in composites with no coatings, and in all cases the shear stress transmission was considerably higher than would be expected from the yield properties of the resin. Measurements of the glass transition temperature of the epoxy resin, as well as Fourier-Transform Infra-Red analysis, indicated modification of resin properties in a zone around the glass fibers. Each of the silane coatings provided more stable thermomechanical properties than those obtained with uncoated glass, at least until the silanes were irreversibly degraded by boiling water. A phenylamino silane provided the most thermally stable properties. Finally, unidirectional E-glass fiber reinforced laminae were fabricated and the measured values of longitudinal strength were compared favorably to theoretical predictions.     

Relationship Between Interfacial Water Layer Adhesion Loss of Silicon/Glass Fiber-Epoxy Systems: A Quantitative Study

Water at the polymer/substrate interface is often the major cause of adhesion loss in coatings, adhesives, and fiber-reinforced polymer composites. This study critically assesses the relationship between the interfacial water layer and the adhesion loss in epoxy/siliceous substrate systems. Both untreated and silane-treated Si substrates and untreated and silane-treated E-glass fibers were used. Thickness of the interfacial water layer was measured on epoxy/Si systems by Fourier transform infrared-multiple total internal reflection (FTIR-MTIR) spectroscopy. Adhesion loss of epoxy/Si systems and epoxy/E-glass fiber composites was measured by peel adhesion and short-beam shear tests, respectively. Little water accumulation at the epoxy/Si substrate interface was observed for silane-treated Si substrates, but about 10 monolayers of water accumulated at the interface between the epoxy and the untreated Si substrate following 100 h of exposure at 24 °C. More than 70% of the initial epoxy/untreated Si system peel strength was lost within 75 h of exposure, compared with 20% loss after 600 h for the silane-treated Si samples. Shear strength loss in composites made with untreated E-glass fiber was nearly twice that of composites fabricated with silane-treated fiber after 6 months of immersion in 60 °C water. Further, the silane-treated composites remained transparent, but the untreated fiber composites became opaque after water exposure. Evidence from FTIR-MTIR spectroscopy, adhesion loss, and visual observation strongly indicated that a water layer at the polymer/substrate interface is mostly responsible for the adhesion loss of epoxy/untreated siliceous substrate systems and epoxy/untreated glass fiber composites and that FTIR-MTIR is a viable technique to reliably and conveniently assess the adhesion loss attributable to water sorption at the interface.     

Strength of Nicalon Silicon Carbide Fibers Exposed to High-Temperature Gaseous Environments

The effects of exposures to high-temperature gaseous atmospheres on the strength of Nicalon SiC fibers were investigated. The exposure conditions were as follows: (1) H₂ with various P _H2O for 10 h at 1000° and 1200°C, and (2) air for 2 to 100 h at 800° to 1400°C. Individual fibers were tested in tension following each exposure. The strengths of the fibers were strongly influenced by the exposure atmosphere and temperature, but less affected by time at temperature. When exposed in air, a SiO₂ layer was formed on the surface, minimizing the degradation of strength. However, this beneficial effect was negated under conditions in which the SiO₂ layer became too thick. The most severe degradation resulted from exposure to a reducing atmosphere, presumably due to the reduction of SiO₂ inherent in the fibers.     

Modulus-Graded Interphase Modifiers in E-Glass Fiber/Thermoplastic Composites

A process for coating E-glass fibers with polystyrene–polyethyleneimine (PEi) core–shell particles was developed, and uniform monolayers of particles of 143 and 327 nm diameter were covalently bonded to the glass surface. The effect of the particle coatings on the mechanical properties of fiber-reinforced composites of poly(vinyl butyral) (PVB) was investigated. The interfacial shear strength (IFSS) was measured for specimens containing one to 20 fibers each using the tensile fiber fragmentation test, and significant enhancements were found, in particular for samples containing larger numbers of fibers. The smaller-particle (143 nm) coatings in the 20-fiber specimens produced approximately a 100% enhancement in IFSS over equivalent specimens with bare or aminosilane-treated fibers, while the 327 nm particle coatings produced only approximately a 25% enhancement. The greater effectiveness of the smaller particles was attributed, at least in part, to the larger effective interfacial area they provide and their relatively greater shell-to-core ratio, providing greater interphase stiffness. The greater enhancements achieved for the multi-fiber vs single-fiber specimens suggest that the coatings produce a more uniform fiber–fiber spacing and, therefore, a more thorough wetting of the fibers by the resin in the multi-fiber samples. Composites formed using fiber tows of 3200 fibers each showed more than a 100% increase in composite toughness and 35% increase in ultimate tensile strength as compared to samples with bare fibers due to the presence of the 143 nm particle coatings, and somewhat more modest increases for the 327 nm particle coatings.     

Development of oriented morphology and tensile properties upon superdawing of solution-spun fibers of ultra-high molecular weight poly(acrylonitrile)

The uniaxial drawing of UHMW-PAN fibers spun from a dilute solution into methanol coagulation baths at different temperatures and the resultant structure and tensile properties of the drawn products were studied. Although the initial morphology of the fibers and the deformation mode in a lower draw ratio (DR_t) range were significantly dependent on the temperatures of the coagulation bath, the tensile properties at a given DR_t, as well as the maximum achieved ones, were comparable. Both the tensile modulus and strength increased steadily with the DR_t and reached 35 and 1.8 GPa, respectively, at the highest DR_t of ∼80. These tensile properties are among the highest ever reported for PAN fibers. The achievement of such high tensile properties for extremely drawn fibers is ascribed to the conformational changes of crystalline chains from the 3/1 helix to the planar-zigzag with increasing DR_t, the improvement in the uniformity of the fiber diameter along the fiber axis, and the decrease in fiber diameter. Indeed, the tensile strength of fibers prepared from a dilute solution and having comparable moduli increased with a decrease in the fiber diameters. The reciprocal of the strength was proportional to the square root of the diameter as suggested by the Griffith theory. Extrapolation to a zero diameter yielded an ultimate tensile strength of 2.4±0.1 GPa for a fiber having a maximum achieved tensile modulus of 35±1 GPa.     

Surface characterization of silane-treated industrial glass fibers

The adsorption of silane coupling agents onto glass fiber surfaces has been investigated. The type of adsorption was elucidated using electron spectroscopy for chemical analysis (ESCA or XPS). The surface charging was recorded using streaming potential analysis. The silane bond strength was tested by boiling the silanized fibers in water for 2 h. Thereafter the conductivity of the water was measured in order to estimate the capability of the silane surface film to prevent ion dissolution from the glass. ESCA provided information on the amount adsorbed and indicated that substantial rearrangement in the surface film structure occurred as a function of the silane concentration. The aminosilane produced a strong positive charge on the glass fibers, while the nonionic silanes were only partly condensed, giving rise to a substantial enhancement of the negative charge. The conductivity measurements indicated that the silane films were present as a loose patchlike silane network on the surface of the E-glass fibers. This conclusion is in accordance with the results obtained with all the techniques used.     

An NMR Imaging Study of the Interface of Epoxy Resin-Glass Fiber Reinforced Composites

NMR imaging was used to analyze E-glass fiber/epoxy composites. The NMR images revealed specific interactions between the glass fiber and epoxy, as indicated by the different degree of curing in the proximity of the fiber. When as-received glass fibers were used, an accelerated degree of curing in the proximity of the fibers as compared with the bulk was observed, whereas when heat-treated glass fibers were used the above trend was substantially inhibited. Finally, when E-glass fibers treated with an aminosilane coupling agent (γ-APS) were used, no variation in the degree of curing in the proximity of the glass fiber was observed. The above results suggest that physisorbed or chemisorbed molecular water present in the surface of the glass fiber assists in the opening of the epoxy ring by hydrogen bonding effects.     

Effects of water on drawability,structure, and mechanical properties of poly(vinyl alcohol) melt‐spun fibers

Poly(vinyl alcohol) (PVA) melt‐spun fibers with circular cross‐section and uniform structure, which could support high stretching, were prepared by using water as plasticizer. The effects of water content on drawability, crystallization structure, and mechanical properties of the fibers were studied. The results showed that the maximum draw ratio of PVA fibers decreased with the increase of water content due to the intensive evaporation of excessive water in PVA fibers at high drawing temperature. Hot drying could remove partially the water content in PVA as‐spun fibers, thus reducing the defects caused by the rapid evaporation of water and enhancing the drawability of PVA fibers at high drawing temperature. The decreased water content also improved the orientation and crystallization structure of PVA, thus producing a corresponding enhancement in the mechanical properties of the fibers. When PVA as‐spun fibers with 5 wt % water were drawn at 180 °C, the maximum draw ratio of 11 was obtained and the corresponding tensile strength and modulus reached ～0.9 GPa and 24 GPa, respectively. Further drawing these fibers at 215 °C and thermal treating them at 220 °C for 1.5 min, drawing ratio of 16 times, tensile strength of 1.9 GPa, and modulus of 39.5 GPa were achieved. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45436.     

An Overview of the Strengthening of Glass Fibers by Surface Stress Relaxation

Pristine glass fiber is well known to become mechanically weaker when heat-treated in the presence of water vapor. However, recently, the same fiber was found to become stronger if heat-treated while held under a subcritical tensile stress at a temperature below the glass transition temperature. The added strength was attributed to the formation of a surface compressive layer on the glass created by a surface stress relaxation process that occurred while being held under the tensile stress in air. Silica glass fibers with strengths estimated to be ~7–8 GPa were produced, exceeding the ~5.5 GPa strength of fresh optical fiber commonly reported at room temperature in air. Similar degrees of strengthening, a 20–30% improvement, have been observed previously for E-glass and is reported here for the first time for soda-lime silicate glasses. This process is a new glass strenthening method that may be applied to all oxide glasses and is not subject to the same constraints as currently available methods.     

Resorbable materials of poly(L-lactide). II. Fibers spun from solutions of poly(L-lactide) in good solvents

Fibers of poly(L -lactide) (PLLA) with a tensile strength up to 1.2 GPa and Young's modulus in the range of 12–15 GPA were obtained by a hot drawing of fibers spun from solution of PLLA in good solvents such as dichloromethane and trichloromethane. The tensile strength of fibers was strongly dependent on the molecular weight of PLLA and on polymer concentrations in the spinning solution. Changing of the polymer concentration in the spinning solution gives rise to formation of fibers with different shape and porosity. Fibers spun from 10–20% solutions at room temperature exhibit a regular structurization, due to the melt fracture. These fibers had knot strengths up to 0.6 GPa, whereas fibers with a smooth surface spun from more dilute solutions had weaker square knots up to 0.3 GPa.     

Microstructural Stability and Mechanical Properties of Directionally Solidified Alumina/YAG Eutectic Monofilaments

Fiber strength retention and creep currently limit the use of polycrystalline oxide fibers in ceramic matrix composites making it necessary to develop single crystal fibers. Two-phase alumina/YAG single crystal structures in the form of monofilaments show that the room temperature tensile strength increases according to the inverse square root of the microstructure size. Therefore, microstructure stability will play a significant role in determining the ‘use temperature’ of these fibers along with its creep resistance. In this work, the effects of temperature on microstructural stability and the creep behavior of directionally solidified alumina/YAG eutectic monofilaments were studied. Microstructural stability experiments were conducted in air from 1200 to 1500°C and creep tests at temperatures of 1400 to 1700°C. Inherent microstructure stability was found to be very good, however, extraneous impurity-induced heterogeneous coarsening was significant above 1400°C. The creep strength of monofilaments with aligned microstructures were superior to ones with low aspect ratio morphologies. Mechanisms for microstructural coarsening and creep behavior are discussed.