Deformation behaviour of PA/PPO blends in relation to microstructure and processing conditions

The use of thermoplastics for structural applications has led to the develo pment of specific blends offering high rigidity over a broad temperature ra nge while displaying efficient energy absorbing mechanisms under impact con ditions. In this respect, engineering plastic alloys like Noryl GTX grades from General Electric Plastics fulfill these criterions. The objectives of this work are to understand how microstructure and processing conditions in fluence the end-use mechanical properties. Three Noryl GTX grades, namely 914, 944 and 964, have been provided by Gene ral Electric Plastics. They consist of a polyamide (PA 6.6) matrix and disp ersed polyphenylenoxide (PPO) phase with roughly 50-50 % composition (fig. 1). A compatibilizing agent is used to create a fine morphology with partic les diameters of 1-2 mu m. The PPO phase is modified by an undisclosed amou nt of elastomer material. Internal microstructure of the PPO particles is c omplex, with the elastomer phase appearing as tortuous domains (fig. 2). In order to simulate the injection moulding of thin body panels, 2 mm thick p lates have been prepared at constant injection speed and mould temperature. The processing parameters considered in this study are melt temperature, h olding pressure and residence time. Injection pressure is also recorded. Ho lding pressure may strongly influence crystallization and frozen stress rel axation upon cooling, while melt temperature affects shear gradients throug h viscosity effects, and may also induce potential thermal degradation in w hich case it should be coupled to residence time. Tensile testing is performed on an Instron machine at constant strain rate. Video extensometry is used to monitor strains in all three directions thus true stress and volume strain are recorded as a function of longitudinal s train. In order to illustrate the drawing behaviour, typical data are prese nted in figure 4 for the three Noryl GTX in standard processing condition. Yielding is characterized by a plateau value at roughly 4 % strain and 50 M Pa stress, followed by a slight hardening. Volume strain exhibits two quasi linear regimes as a function of longitudinal strain, with a higher slope i n the first regime. The cavitation rate, i.e. d(Delta V.d epsilon), taken i n the first cavitation regime has been retained to probe the influence of m icrostructure and processing conditions on the deformation behaviour of the se blends. Yield stress is a function of material structure (fig. 5 and 6). Blending o f PPO/elastomer into PA matrix reduces the yield stress owing to multiplica tion of stress concentration sites encouraging plasticity. Noryl GTX 914, f or which the nature of the elastomer phase is different of the other grades , shows higher yield stress and hardening under similar processing conditio ns. The drawing behaviour of Noryl GTX 944 and 964 is roughly similar. No particular correlation is obtained with either residence time or holding pressure, whereas melt temperature enables some discrimination of the proc essing conditions. However, the most striking result is shown in figure 11, which relates the cavitation rate to the injection pressure. This paramete r is not a controlled variable and is a complex function of temperature and shear gradients. Noryl GTX 964 exhibits a linear variation of the cavitati on rate as a function of injection pressure. Surprisingly, a reverse trend of evolution is observed in the case of GTX 914. Morphological studies have been undertaken, using both scanning electron mi croscopy and transmission electron microscopy, in combination with contrast ing techniques. First of all, blend characteristics have been derived in re lation to the various processing conditions. Neither particle size nor part icle size distribution appear significantly altered, even under somewhat ab usive conditions. SEM investigations on fractured surfaces after tensile dr awing reveal a distinct behaviour of the different blends (fig. 7 to 9). No ryl GTX 914 fracture surfaces offer a rather compact morphology, with a cav itational process essentially linked to particle-matrix interface rupture. On the contrary; Noryl GTX 964, and to some extent 944, show a complex fibr illated structure associated with elastomer cavitation and intense drawing of the inner particle PPO ligaments. Profuse fibrillation of the PPO partic les is thus the key to the observed deformation behaviour of these blends. The key role played by the elastomer phase is emphasized in the comparison made in figure 10 of the typical drawing behaviour of a PA/PPO blend withou t rubber with that of Noryl GTX 914. The drop in yield stress accompanied b y a raise of cavitation in GTX 914 underlines the essential contribution of the elastomer phase to trigger the plasticity of the PPO phase. By analyzing the plastic deformation of PA/PPO blends, it has been possible to relate the extent of interparticle cavitation to a rather complex param eter of the injection process, the injection pressure. Depending on the nat ure of the elastomer phase in these blends, a reverse trend of evolution is observed when going from standard to severe processing conditions. Inner P PO particle microstructure thus appears as a key element for achieving toug hness, as long as the PPO - matrix interface remains coherent. The higher c apability of Noryl GTX 964 as compared to GTX 914 regarding intraparticle f ibrillation is in favour of higher toughness.