Steven B. Warner

Electrospinning and Properties of Some Nanofibers

Electrospinning is a unique process that is capable of producing fibers with diameters ranging over several orders of magnitude, from the micrometer range typical of conventional fibers down to the nanometer range. Electrospun fibers possess unusually large surface-to-volume ratios and are expected to display morphologies and material properties different from their conventional counterparts. In this paper, details of recent designs and construction of equipment for controllable and reproducible electrospinning or electrostatic spinning are presented. An understanding of the electrospinning process is linked to processing conditions and polymer fluid characteristics, as well as the structure and properties of the final electrospun nanofibers.

The formation and performance of auxetic textiles. Part I: theoretical and technical considerations

Auxetic textiles belong to a class of extraordinary materials that become fatter when stretched. Sustained efforts to fabricate auxetic fabric structures are limited. Indeed, several geometrical configurations have been previously proposed but none has been engineered into functional auxetic textile fabrics. The use of auxetic materials has been limited because of problems with deploying them in their fabricated forms. Our thrust in this research is to combine our knowledge of geometry and fabric structural characteristics to engineer auxetic textiles and to determine the properties of such auxetic textile fabrics. In this paper, we have presented the technique we developed for producing several knit structures in which filling yarn inlays are used to effect compound repeating units. In these productions, the chain is used as a base structure and a minimum of two guide bars and maximum of six guide bars are deployed to produce such warp knit auxetic fabrics.

The formation and performance of auxetic textiles. Part II: geometry and structural properties

Some exceptional materials become fatter when stretched and are described as auxetics or having negative Poisson’s ratio. Auxetic textiles belong to this class of extraordinary materials that are increasingly attaining some prominence in many applications of technical textiles. We have sustained the efforts to fabricate auxetic fabric structures based on non‐auxetic yarns. The focus is to combine our knowledge of geometry and fabric structural characteristics to engineer auxetic textiles and to determine the properties of such auxetic textile fabrics. To realize our objective, we designed and investigated hexagonal knit structures as auxetic textiles offering optimum performance. The factors that influence Poisson’s ratio are identified as yarn type, number of chain courses and strain level. Also, a method has been developed for quantifying the geometrical structural unit cell of the auxetic structure based on measured parameters, namely a 1, a 2, b 1, b 2, h and c, as detailed in this paper.

OPTIMIZATION OF ELECTROSPINNING PROCESS PARAMETERS FOR TISSUE ENGINEERING SCAFFOLDS

The goal of the current study was to optimize important process parameters for electrospinning polycaprolactone (PCL) for growing 3T3 fibroblasts. We hypothesized that the smallest obtainable fiber diameter would provide the best cell growth kinetics and we tested this hypothesis for three different process parameters: solution concentration, voltage and collector screen distance. Beaded structures were formed when using low concentration electrospinning solutions (8 wt% to 13 wt%), in which the viscosity ranged from 16.0 cP to 340.0 cP. In this concentration range, cell growth kinetics was impeded when using a high concentration of cells (8–10 × 105). Higher PCL concentration led to an increase in the average fiber diameter from 400 nm to 1600 nm when PCL solution concentration changed from 15 wt% to 20 wt%. Although, the mean values indicated that cell growth kinetics were higher at the lower end of the concentration (15% as opposed to 20%) and this correlated with lower average fiber diameter, the results in this range were not statistically significant (p > 0.05). The average fiber diameter of scaffolds first decreased and then increased when electrospinning voltage was increased. The cell growth kinetics demonstrated that smaller average diameter PCL fiber scaffolds had higher growth kinetics than larger average diameter scaffolds with the best conditions obtained at 15 KV. By increasing the screen distance, the average fiber diameter decreased but had no significant impact on cell growth kinetics. In summary, the optimal parametric space for 3T3 fibroblast growth for our studies was electrospinning a 15 wt% PCL solution using 15 kV voltage and a 25 cm collector distance.

Adhesive point-bonded spunbond fabrics

Adhesive technology has improved enormously in the few past decades, so adhesive point-bonding of spunbond fabrics has been given a fresh look to ascertain whether thermal point-bonding may have a new challenge. Finite element analysis is implemented to develop a simple truss-based model capable of predicting the tensile behavior of adhesive point-bonded spunbond fabrics on the basis of the fibers, a judiciously chosen adhesive, and the web properties. Pattern-bonded polypropylene spunbonded fabrics are fabricated using a screen printing technique and tested. Stress-strain curves generated by finite element analysis agree with the experimental results. Tensile properties of the adhesive-bonded fabrics are significantly better than those of the same thermal point-bonded fabrics, even without optimizing the adhesive. Roll screen-applied UV rapid-cure adhesives may be used to develop a practical method of production.

Fibrous Structures with Designed Wicking Properties

Fibrous structures have been designed and tested to achieve anisotropic flow properties. A structure that transports liquid in one specific in-plane direction as fast as possible was formed by parallel arrangement of continuous filament yarns in the middle layer of a three-layer structure. The top and bottom layers consisted of low areal density nonwoven fabrics. Tests showed that flow was faster in the middle layer, yielding a nonuniform flow front among the layers. Structures with larger pores in the middle layer wicked faster, as anticipated from the Washburn equation. Although we maximized the in-plane orientation to increase the flow anisotropy of the structures, the flow anisotropy was only approximately 3, which can be usefully compared to a mechanical anisotropy of the structure of perhaps 50 or higher. A mathematical expression derived from combining the continuity equation with Darcys Law was used to model the flow behavior in these structures. The predicted values of the flow front were within 20% of the experimental values. Another structure was designed to maximize liquid transport through the thickness of a fabric. It contained flocked fibers in the middle layer oriented parallel to the direction of flow. Structures with single and double layers of flocked fibers with varying fiber denier and flock density were made and tested. The number of layers, the fiber denier and the interaction between the number of layers and flock density influenced the transverse flow behavior of these structures. Single layer samples with high-flock density and high fiber denier promoted transverse flow best. Double layer high-flock density samples hindered the transverse flow, retaining the liquid in the middle region where the layers met to produce smaller pores.

Penetration of Blade-Applied Viscous Coatings into Yarns in a Woven Fabric

We developed a model based on first principles — Darcy’s law and lubrication theory — that can be used to predict the intra-yarn penetration depth of a coating applied to a fabric using the knife-over-roll technique. We verified the model by studying the penetration of a polyurethane coating formulation onto a woven nylon substrate. The results show that the penetration depth was greatest, 41 µm, when the gap between the blade and the substrate was smallest; however, we varied application speed and coating viscosity as well as gap distance. The minimum penetration depth was 18 µm, or about one fiber diameter, as determined by analysis based on scanning electron micrographs. Although the calculated penetration depths were about three times less than the measured depths, at least in part because of the many approximations and assumptions required by our derivations that are based on first principles, we consider the agreement to be good. The work brought out another important result: neither viscosity nor coating speed affected penetration depth.

Modeling the Bending Stiffness of Point Bonded Nonwoven Fabrics

A simple method for predicting the bending behavior of point bonded nonwoven fabrics has been developed. Proven theories found in the literature have been combined using elementary geometry and probability to produce a method to calculate the bend ing rigidity of nonwoven fabrics based on the properties of the constituent fibers and the fabrics construction. Pattern-bonded polypropylene and polyester fabrics, both spunbonded and bonded carded web, have been analyzed. The theoretical results agree with experimental values of bending stiffness.

The Strength of Thermally Point-Bonded Nonwoven Fabric

A semi-empirical model is developed to predict the maximum achievable strength of a nonwoven fabric and to identify and scale the factors affecting that strength. The scope and accuracy of the model are limited by the assumptions made during its development, but they are shown to be quite reasonable. The results of the model calculations suggest that the practical potential strength of an ideal point-bonded nonwoven fabric is three to four times higher than the average tensile strength of the best available spunbonded fabric samples, and about one-third lower than the average strength of fiber samples. Weak fiber-bond interfaces, areal density variations, fiber property variations, and fiber curl or combinations of these and others factors may limit fabric strength. Of these four factors, the first two are the key ones that limit spunbonded fabric observed strength. Knowledge of the potential strength and the factors limiting strength may enable manufacturers to improve nonwoven strength.

The Melt Electrospinning of Polycaprolactone (PCL) Ultrafine Fibers

Electrospinning is a technique of producing nanofibers from polymer solution/melt solely under the influence of electrostatic forces. In this research, we investigated the formation of nanofibers by melt electrospinning polycaprolactone (PCL). The effect of process parameters such as molecular weight, applied voltage, and electrode separation on the fiber diameter was investigated. Controlling the process parameters could help increase the proportion of ultrafine fibers in the melt electrospun nonwoven mat. The velocity of the straight jets was in the range of 0.2-1 m/s. The melt electrospun fibers were characterized with respect to fiber diameter, distribution, mechanical properties and birefringence. Melt electrospun polycaprolactone fibers had a diameter distribution of the order of 5 -20 μm. The birefringence of the melt electrospun fibers increased with decrease in fiber diameter.