Direct determination of the orientation of short glass fibers in an injection‐molded poly(ethylene terephthalate) system

Abstract A simple and new technique for direct determination of the short fibers orientation in polymer composites is described. It is based on the fact that the shape of the fibers' cross section in the plane perpendicular to the desired direction (e.g., orientation direction) depends on the orientation angle ϕ with respect to this direction. For an eliptical shape with minor and major radii r and R, the cosϕ = r/R. Using micrographs from the polished samples' cross section and assuming a planar fiber distribution, it is possible to calculate for a part or entire cross section area: (i) the ϕ‐values; (ii) the orientation parameter f_p; (iii) the angular distribution (by histograms); and (iv) the number and the sizes of the layers with more uniform fiber orientation as well as the transition zones between them. It is demonstrated for commercial poly(ethylene terephthalate) (PET) reinforced with 45 percent (by weight) short E‐glass fibers (Rynite®) that each surface layer (A) takes ca. 3/16 of the sample thickness (b), the core (C)—3/16b and the boundary (B) between them occupies 3/16b. These layers are characterized, respectively, by <ϕ>_A = 28°, <ϕ>_B = 38°, <ϕ>_C = 64°, and f = 0.51, f = 0.18, f = −0.15. For the entire thickness (A + B + C) the <ϕ>_A+B+C = 44° and f = 0.17. The method described is applicable to any kind of fiber‐reinforced composites from which it is possible to obtain micrographs of a desired cross section. The thickness of the cross section studied, as well as the number of the layers, does not affect the efficiency of the method.