JEC COMPOSITES MAGAZINE - Issue #33 - June 2007 - 64

Properties of long-fibre-reinforced thermoplastic composites KNOW-HOW Stretch-broken fibres GUY BONTEMPS R&D MANAGER, The use of long stretch-broken fibres raises questions about the resulting mechanical properties of the final composite. Are there fundamental differences between this type of fibre concept and the more widely known continuous-filament systems? What advantages are there to the process? In this article, we will try to answer these questions. Stretch-breaking technology SCHAPPE TECHNIQUES XAVIER GIROUX, SALES MANAGER The drafting gradient is inversely proportional to the distance between nipping points. We also note that the number of drafting zones required to stretch-break a fibre tow varies proportionately with the maximum ultimate elongation of individual fibres. Among the many different processes used in the textile industry to turn continuous filament tows into discontinuous staple fibres, stretch-breaking stands out because of its apparent simplicity and its capacity to keep all the fibres in their initial order. This is due to the stress modes that affect the fibres. In the stretch-breaking process, only tensile stress is concentrated along the fibre axis in an area called the drafting zone. This is controlled by a pair of feed rollers with a tangential speed of Vo, and a pair of drafting rollers with a tangential speed of V1. These speeds apply to the fibre bundle perpendicular to the fibre nipping point (the theoretical tangent point for the rollers). A drafting gradient affects every point in the drafting zone, and therefore, every point and every fibre in the fibre bundle. A machine consists of a succession of drafting zones; starting with the second of these zones, each pair of rollers assumes both feed and draft functions. JEC Composites Magazine / No33 June 2007 Fibre breaking mechanism Any textile fibre has discrete flaws, structural defects, or local impurities linked to the polymer chemistry, the spinning process or, in the case of carbon fibre, the oxidation/carbonisation process. As a result, randomly distributed breaks will occur at these points of high stress concentrations. Thanks to this property, it is possible to stretch-break textile fibres. If all the fibres were perfectly isotropic, pure and free of defects, the probability of break would be the same for all points on each fibre, making the stretch-breaking process impossible. Characterising a fibre population Let us point out that the product of stretch-breaking is a continuous sliver of discontinuous fibres that are held together “side by side” using one of two different techniques as a function of fibre type: - For organic and metal fibres, the cohesiveness of the fibre A draft rate (e1, e2) is defined, which is the ratio of the speed of the drafting rollers to the speed of the feed rollers. The ratio is obviously more than 1 and characterises the draft rate in the zone under consideration. Another important parameter is the distance (l) between nipping points for the feed and draft rollers. This distance determines the median height for fibre-length distribution and the length of the longest fibre in the distribution.

JEC COMPOSITES MAGAZINE - Issue #33 - June 2007

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #33 - June 2007