Rheology of Non-Newtonian Fluids Containing Glass Fibers: A Review of Experimental Literature

The objective of this review is to elucidate the rheological behavior of glass fiber suspensions whose suspending mediums are non-Newtonian fluids. In particular, this review focuses on determining the impact of fiber concentration, aspect ratio, orientation distribution, interaction with the suspending medium, and suspending medium viscoelasticity on the rheology of glass fiber composite fluids. The presence of glass fiber can induce a yieldlike behavior, causing shear thinning to occur at reduced shear rates. Glass fiber can impede the elastic properties of the suspending medium but enhance the first normal stress function. Large stress overshoots in both the shear and normal stress growth functions are observed that are associated with changes in fiber orientation. Upon cessation of flow, stress relaxation follows that of the suspending medium but fibers retain their orientation. The presence of glass fiber can induce extension rate thinning and suppress the strain thickening behavior of the suspending medium. 1. Introduction Glass fibers have been used for decades to improve the mechanical, thermal, and insulative properties of polymers. 1 These property improvements are highly dependent on the orientation distribution of the glass fiber. This makes it desirable to be able to predict not only the rheological behavior of the composite fluid but also the orientation of the fiber generated during processing. Understanding the rheological behavior of polymeric fluids containing glass fibers is essential to model development. With respect to the fiber, it is of interest to understand the role of concentration and aspect ratio and their relation to the degree of interparticle interaction (fiber -fiber) as well as orientation distribution and interaction with the suspending medium. With respect to the suspending medium, it is of interest to understand the role of viscoelasticity and how it is affected by the presence of the fibers. The rheological properties of glass fiber suspensions in Newtonian suspending mediums has been reviewed in detail by Ganani and Powell 2 and Zirnsak et al. 3 and will be referred to in this review for comparison purposes only. The primary focus of this review is to elucidate the rheological properties of glass fiber suspensions in non-Newtonian fluids of various degrees of viscoelasticity with an emphasis on composite fluids. Before reviewing the rheology, it is imperative to have a basic understanding of glass fiber suspensions, the rheometers used to characterize them, and any extra forces that can lead to mechanisms for changes in the fiber microstructure. For this reason, subsequently, we will briefly discuss the use of surface treatments to increase the interaction with the matrix. We then classify fiber suspensions by their concentration and length. This is followed by a review of the different rheometers and rheometer geometries used to characterize glass fiber suspensions, including a discussion of their strengths and weaknesses with respect to obtaining accurate measurements of rheological material functions. Last, we discuss and estimate the contribution of Brownian motion and gravity (relating to particle sedimentation) to changes in fiber orientation within the suspension. 1.1. Surface Modifications. The surface of a glass fiber is typically modified for two reasons: to minimize the selfdestructive abrasive contact between the fibers and to increase the fiber-matrix interaction of the melt and the adhesive strength of the composite. The use of “sizing” as a surface treatment has become an industrial standard and addresses both the abrasive contact and the surface interaction. Sizing is a functional coating that acts as an abrasive barrier that is tailored