Direct study of the structure and properties of transcrystalline layers

Abstract

When a fibre is embedded into a thermoplastic melt it may act as a nucleant for the growth of spherulites. If there are many nucleation sites along a fibre's surface, then the resulting spherulite growth will be restricted in the lateral direction, so that a columnar layer, known as transcrystallinity [1] will develop and enclose the fibre.The presence of anisotropic layers, such as transcrystallinity, has significant implications for the performance of fibre-filled thermoplastics composites. Several workers [2][3][4][5] have examined this topic in composites, without fully characterizing the mechanical properties of the transcrystalline layer. As it is very difficult to establish the properties of such layers in situ in a fibre-reinforced composite, it was decided that the best approach for characterizing the microstructure and properties of the transcrystalline layer would be to study it in isolation, as a two-dimensional analogue.A way of creating such a two-dimensional analogue would be to prepare totally fiat transcrystalline sheets of material. One method of achieving this could be by sandwiching thin layers of a suitable polymer, such as polypropylene, between sheets of a nucleating material, such as poly(ethylene terephthalate) in the form of a Melinex sheet. It should then be possible under suitable thermal conditions to obtain transcrystallinity in isolation. For comparison purposes one could also make polypropylene sheets of a differing microstructure by appropriate modifications to the thermal conditions employed during the manufacture of the sheet.For our studies, the polypropylene chosen was the I.C.I. grade GW522M, which has been previously shown [6] to be capable of displaying thick (up to 150#m) and regular transcrystallinity against poly-(ethylene terephthalate) when in the form of Terylene fibres. A typical thickness of a transcrystalline zone is of the order of 50 to 75/~m, so our aim was to produce polypropylene sheets 100 to 150 #m thick.Accordingly, a tape of polypropylene, of 100 to 150/~m thick, was first extruded using a Plasticiser Ltd Fibrillator. Secondly, the tape was sandwiched between two sheets of Melinex, which was then held, in compression, between two glass plates. This arrangement was then kept in an oven held at 200°C for 30 min.In order to produce transcrystallinity, the system was transferred to a separate oven held at 120°C for a further 10 min, after which the tape was quenched in cold water. In addition, a fine spherulitic sheet was prepared by directly quenching the molten tape in cold water.For the microstructural studies, the two types of polypropylene sheet were microtomed, in cross- section. For this purpose, the sheets were stacked together, as six to eight small pieces on to a cold stage, and then cut into 20 to 40/~m thick sections using a steel blade. They were subsequently examined "edge on" under crossed-polars using a Reichert Zetopan microscope. Fig. 1 shows a particularly well-developed transcrystalline morphology, where two transcrystalline layers have gro...

On the origin of transcrystalline morphology in polymers and their composites: Re-evaluation of different views

“…Moreover, the frequent presence of transcrystalline layers on the external surfaces of polymers crystallized from melt [19,27,28] indicates that this particular morphology found on the boundary is related to the faster cooling due to a different heat transfer mechanism (such as convection rather than conduction) or, for thick specimens, to the direct contact with the cooling system. Indeed, because of the temperature gradient along the thickness, the morphology of polymers varies from the external surfaces to the interior part of specimens [1,10,17,29,30].…”

Hardness and Young's modulus of transcrystalline polypropylene by Vickers and Knoop microindentation

“…Moreover, the Knoop hardness values are systematically higher because of the difference in the numerical coefficients in eqs. (1) and (2). However, anisotropy ratios for the Vickers and Knoop may be defined and compared.…”

“…Upon cooling from the melt, heterogeneous nucleation results, followed by crystal growth normal to the fiber surface and restricted in the lateral direction, so that a columnar layer develops and encloses the fiber. 2,9 There is some controversy in the literature 10 -15 as to whether or not the presence of transcrystallinity has an effect on load transfer, toughness, and on the thermal residual stresses in polymer composites, and to what extent. To resolve this controversy, it may be necessary to know the anisotropic mechanical properties of the TC region.…”

Particulate Fillers in Thermoplastics

“…A large variety of materials are used as fillers in composites. Besides CaCO3 and carbon black (see Table 1) a large number of other materials like mica [63,73,95], short [96][97][98] and long glass fibers [99,100], glass beads [101][102][103][104][105][106][107][108][109][110], sepiolite [38][39][40][41][42][43][44][45][46][47][48][49], magnesium and aluminum hydroxide [111][112][113], wood flour and cellulose [30][31][32][33][34][35][36][37][115][116][117], wollastonite [102,[118][119][120], gy...…”