TECHNOLOGY | REACTIVE COMPOUNDING
The IKT and DITF researchers used a specially- modified co-rotating twin screw extruder with a clamshell-type openable cylinder to observe the reaction process of a linear chain extender with the flame-retardant polyamide over the whole length of the processing section. Rheological measure- ments proved that the viscosities of the produced compounds could be increased to the level of standard polyamides. Further testing has con- firmed that the compounds can be processed via injection moulding.
Above: IKT is researching ways to use reactive extrusion to bind flame retardant groups to PA polymers
This has negative effects on the mechanical properties of the polyamide or can lead to a surface migration if the additive is physically bonded to the polymer chain.” For this reason, the IKT has been cooperating with the German Institute for Textile and Fibre Research (DITF) in Denkendorf on a way to chemically bind additives to the polyamides, along with their compounding and processing. “The chemical binding can be achieved by means of binding the flame retardant into the polymer chain during the polyamide synthesis. However, after completing the synthesis, the flame retardancy molecules occupy many of the polyamide end groups. This in turn impedes the chain growth and leads to low molecular weights and low viscosities,” she explains. “As a result, these intrinsically flame-retardant polyamides can be processed into fibres very well, but they are unsuitable for classical plastics processing methods like injection moulding. To increase the viscosity and to enable more process- ing methods, a reactive extrusion with linear chain extenders like epoxides, oxazolines or (bis) caprolactames is helpful,” she says.
Flame performance In addition, Pagel says, a flame test showed that the chain extension did not affect the flame retardance. “The burning durations were significantly below the values of a standard-polyamide. Therefore, the IKT was able to provide the new intrinsically flame-retardant polyamide access to typical plastics processing methods,” she says. Reactive extrusion with molecular weight additives is possible with various polymers, including polyamides, and also with some biopoly- mers, most notably polylactide (PLA). “Due to the similar properties of polylactide to polystyrene, in the long term, polylactide might replace PS-based foams,” Pagel says. “For now, the production of PLA foams with a low density and consistent cell morphology is difficult to realise. This is because of its low melt viscosity and strength as well as its sensitivity to hydrolysis and low crystallisation rate.” Working with the Department of Polymer Engineering (PE) at the University of Bayreuth, IKT has researched the influence of material properties on modification for foaming and on the characteris- tics of the final foam. Initially, the thermal and rheological properties of various PLAs were investigated with the focus placed on the molecu- lar properties of the different PLA types – chain length, number of reactive end groups, and isomeric ratio, for example.
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PHOTO: SHUTTERSTOCK
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