The influence of processing on fiber orientation and creep in short carbon‐fiber reinforced low density polyethylene and polycarbonate

Abstract

This paper is concerned with the creep behavior of short fiber‐reinforced thermoplastics, especially with regard to the role of fiber orientation. Rectangular samples of low density polyethylene (LDPE) and polycarbonate (PC) containing varying amounts of carbon fibers were prepared by compression and injection molding. The materials were compounded using a technique producing a concentration independent fiber length distribution. The orientation distribution, on the other hand, was found to be strongly influenced by fiber concentration. The creep parameters were measured for both LDPE and PC. The contraction ratio was determined for the PC samples. In the case of LDPE reliable data could not be obtained due to the low modulus of the matrix. The creep properties of the PC and LDPE samples varied significantly with the orientation of the fibers. The creep strain was measured as a function of time for both polymers for different fiber concentrations and orientations. The predictions of the Halpin‐Tsai equation underestimated the experimental strain figures somewhat when the stress direction coincided with that of the fiber orientation. When the stress acted across the fibers the theoretical and experimental results showed satisfactory agreement. A plausible explanation is that incorporation of carbon fibers changes the morphology of the matrix material. We conclude that the contraction ratio is an important measure of the volume and its changes during deformation, especially with regard to its relation to the free volume and similar quantities. We have shown that the necessary data can be obtained, in spite of experimental difficulties even for anisotropic samples.

Effects of Fiber Length and Orientation Distributions on the Mechanical Properties of Short-Fiber-Reinforced Polymers

“…An extensive research effort has been conducted to evaluate fiber breakage during processing. A number of techniques have been developed for investigating fiber length degradation in compounded and molded materials [5,6,9,11,14,19,23,24,28,29,32,55,56]. In all the techniques employed, fibers must be separated from the polymer matrix.…”

“…In all the techniques employed, fibers must be separated from the polymer matrix. The polymer matrix may be removed by burning off in an oven [5,6,9,11,19,23,28,29,55,56] or by dissolving the polymer in an appropriate solvent [14,24,32]. It has been shown that fiber breakage in processing of reinforced thermoplastics results from fiberpolymer interaction, fiber-fiber interaction, and fiber contact with surfaces of processing equipments [24].…”

“…The fiber orientation in plaque-shaped parts was studied theoretically and experimentally and a skin-core layer structure was recognized [7,29,31,32,36,37,46,59]. In the skin layer the fibers align preferentially along the flow direction; in the core layer the fibers align transverse to the flow direction.…”

“…Usage of low mold temperature can lead to a comparatively large skin thickness [32]. The FOD in injection molded short-carbon fiber-reinforced composite plaques was shown to be strongly influenced by fiber content [29]. At high fiber contents fiber-fiber interaction plays an important role as a factor affecting and fibers and the fiber radius, respectively.…”

The effect of short fiber orientation on long term shear behavior of 40% glass fiber reinforced polyphenylene sulfide

“…By increasing the fiber fraction, the material response magnitude increases (i.e., increase in relaxation modulus or decrease in creep compliance), and material time-dependency decreases (decrease in relaxation or creep rate), since the fibers may be obstructing the polymer's molecular mobility [7][8][9][10][11]. It's important to mention i.)…”