Ludwig Rebenfeld

A microbond method for determination of the shear strength of a fiber/resin interface

Abstract The shear strength of the fiber/resin interface in a fiber-reinforced composite is usually determined by measuring the force needed to pull a single fiber axially out of the solid matrix. To make such pull-out measurements, however, the length of embedded fiber must be small enough so that the fiber does not break before it pulls free. This is difficult to achieve by conventional methods with fibers of small diameter, such as the high-strength glass, Aramid, and carbon fibers now extensively used in practice. The approach taken is to deposit a droplet of the matrix resin on the fiber and to support the cured droplet appropriately during pull-out. Successful shear debondings are obtained in most trials, and the nature of the recorded force curves during the trial distinguishes between proper shear debonding and slipping of the droplet or fiber breakage. Interfacial bond strength results for an epoxy resin and several fiber types are adequately reproducible and are reasonable in light of the nature of the fiber surfaces, indicating that the microband method may be generally applicable.

Radial penetration of a viscous liquid into a planar anisotropic porous medium

Abstract The equations describing the radial encroachment of a viscous liquid into a homogeneous, anisotropic porous medium are formulated and solved by two approximate methods. An analytical approximation is in good agreement with a finite element numerical solution, provided the angular component of the pressure gradient in an elliptical coordinate system is small. In the specific case where one of the principal flow directions is perpendicular to the flow plane, treatment of experimental flow data in accord with the analytical approximation determines the principal in-plane permeabilities and the degree of in-plane anisotropy. In the general case, the analysis yields effective permeabilities that are functions of the principal permeabilities and the orientation of the principal coordinate system.

In-Plane Flow of Fluids in Fabrics: Structure/Flow Characterization

A technique that quantifies the in-plane flow properties of fibrous networks is pre sented. A radial flow geometry allows the simultaneous study of more than one in- plane flow direction. The shape and position of a radially advancing fluid front define directional permeabilities in the plane, which can be used to compare structurally different fibrous networks. Confirmatory studies reinforce the method of data analysis and show that the directional permeabilities are independent of driving pressure, fluid viscosity, and fiber surface wettability properties. The methodology was applied to the study of multilayer assemblies. The in-plane flow of fluids in heterogeneous multilayer constructions is governed by the high permeability layer. A transverse flow mechanism is proposed as being responsible for filling the low permeability layers and, therefore, the transverse permeability is coupled into this flow process.

Interactions between Solvents and Polymers in the Solid State

Abstract One of the goals of modern polymer research is to achieve an understanding in rigorous and quantitative terms of the molecular chain configuration and of the spatial arrangement of the polymer molecules in the solid state. This problem is of particular interest for oriented semicrystalline polymers such as textile fibers which have both crystalline and amorphous domains or phases. This duality in structure has found expression in many models including the micellar concept, the fringed micellar concept, and fringed fibrillar concept, and more recently the crystalline imperfection model. While there is no universal agreement about an exact description of the fine structure of semicrystalline polymers, it is generally accepted that some of the chains or chain segments are preferentially oriented and aggregated by close packing with neighboring molecules into cohesive structural units which are usually referred to as crystallites. Polymer chain folding is frequently involved in the development of cry...

DYNAMICS OF WATER VAPOR TRANSMISSION THROUGH FABRIC BARRIERS

An experimental apparatus has been developed to permit the simultaneous measurement of moisture sorption by a fabric and moisture flux through a fabric during the transient period after the fabric is exposed to a humidity gradient. By suitable manipulation of the experimental conditions, the method demonstrates that moisture flux occurs predominantly through the air spaces of the fabric; the role of the fibers is to act as a moisture source or sink. Two mathematical descriptions of the transient period have been developed, in which the interaction between moisture sorption by the fibers and moisture flux through the air spaces of a fabric is modeled. In the first model, the dominant mass transfer resistance for the sorption process is considered to be diffusion of water molecules through the air to the fiber surface. Diffusion within the fiber is considered to be so rapid that the fiber moisture content is always in sorptive equilibrium with the air at the fiber surface. This simple model correctly predicts the qualitative trends exhibited by experimental data collected on fabrics of widely varying sorptive capacity, but it under-predicts the duration of the transient period for a large proportion of these fabrics. A more realistic and complex model is also presented, in which the dominant mass transfer resistance is molecular diffusion of water molecules within the fiber interior. In this case the fiber moisture content lags behind the changes in the moisture content of the air at the fiber surface. The added complexity of this model provides a more accurate fit to the experimental data, although the correction provided by accounting for intrafiber diffusional resistance is small compared to the effect of sorptive capacity.

Interactions of Nonaqueous Solvents with Textile Fibers Part VII: Dyeability of Polyester Yarns after Heat and Solvent-Induced Structural Modifications

Pretreatment of polyester yarns with a strongly interacting solvent (dimethylformamide) leads to modifications of the fiber structure which permit rapid diffusion of even “high-energy” disperse dyes under atmospheric conditions without the addition of carriers. A comparison of the effects of solvent pretreatments with the effects of thermal pretreatments on the dyeing behavior has been carried out. Pretreatment in a strongly interacting solvent leads to a high degree of swelling and at higher temperature to the formation of crystallites within the swollen structure. It appears that the swollen structure can be stabilized, depending on the size and stability of the crystallites formed, leading to cavitation and void formation upon subsequent removal of the interacting medium. It is suggested that a rigid pore mechanism of dye diffusion becomes operative in this structure, as opposed to the free volume mechanism of diffusion in thermally-treated polyester yarns.

Interactions of Nonaqueous Solvents with Textile Fibers: Part IV: Effects of Solvents on the Mechanical Properties of Various Textile Yarns

In order to provide a rational basis upon which future developments in nonaqueous finishing and other processing of textile materials may be based, a basic study is being undertaken of the interactions between textile fibers and a wide spectrum of organic solvents. In this paper are reported the effects on mechanical properties of a polyester yam of 26 organic solvents differing widely in chemical and physical characteristics. The yams were conditioned in each solvent for 16 hr at 21°C and their load-extension behavior determined in the solvent. While many solvents were found to have only minor effects on the mechanical properties, several solvents caused major changes in the shape of the load- extension curve of the polyester yam. The active solvents caused a decrease in the initial modulus, a decrease in the yield stress, and an increase in extensibility. In several cases a significant plastic flow region could be noted. Dioxane, acetone, trichloroethylene, tetrachloroethane, methylene chloride, nitrobenzene, nitromethane, acetonitrile, and di methylformamide (DMF) were among the most active solvents. These solvents also caused a major irreversible longi tudinal shrinkage of the yarn, and the mechanical properties of the yarn were only partially recovered after removal of the solvent and reconditioning to a standard condition. The interactions between the solvents and the polyester yarn, as estimated by the decrease in the yarn elastic modulus, were correlated with the solubility parameter δ of the solvents. It was observed that maximum interactions take place with solvents whose solubility parameter approaches that of polyethylene terephthalate δ=10.7. There is some indica tion that these strongly interacting solvents fall into two groups: one with δ values ranging from 9 to 10, the other with δ values ranging from 11.5 to 13. It is suggested that these ranges may reflect specific solvent interactions with the aro matic and aliphatic segments of the polyester molecules.

Physicochemical Properties of Sized Yams Part I: Initial Studies

Several methods were explored for characterizing sized yams and size films in order to establish relationships that might be useful in predicting weaving performance. A variety of natural and synthetic sizes were applied to cotton and to polyester/cotton spun yams in a laboratory sizing apparatus. Although yam tensile tests indicate that sizing generally increases yam strength and decreases yarn extensibility, these tests were inadequate for differentiating among sizing agents. Important differences among the various sizes and levels of size add-on could, however, be detected by measuring rupture lifetimes of the treated yarns using the TRI cyclic tensile abrader, which imposes simultaneous tensile, bending, and abrasive stresses. Two factors appear critical to size performance: the size must form a tough but flexible film to protect the yam from flexing and rubbing actions, and there must be good adhesion between size and yam. The flexibility requirement was established by correlating the tensile behavior of films prepared from the various size polymers with rupture lifetimes of the sized yams. The degree of adhesion was evaluated by measuring the wettability of the size films, using an adaptation of the Wilhelmy wetting force method, and matching the resulting contact angle hysteresis patterns.

Relation between the X-Ray Angle of Cottons and Their Fiber Mechanical Properties

Correlations are presented between the spiral angle, as measured by the X-ray angle (40% absorption angle of 002 diffraction are), and the mechanical properties of 16 cot tons. A significant correlation is found for Pressley strength at zero gauge length : however, virtually no correlations exist when cotton strength is measured at 5.0-mm. gauge length. Highly significant correlations are found between the spiral angle and the extensibility properties of cotton fibers, as measured by single fiber elastic modulus and elongation to break.

Moisture Induced Changes in Fabric Structure as Evidenced by Air Permeability Measurements

Air permeability measurements over the range of 0 to 95% relative humidity indicate that changes in fiber dimensions caused by moisture absorption lead to changes in fabric structural parameters such as thickness and porosity. This phenomenon, which was most noticeable in the more hygroscopic fabrics, was observed in both woven and nonwoven fabrics. Important variables include the fabric structure and intrafabric constraint factors, such as number of bonding points, yarn twist, and yarn cross-over points.