SynopsisExtruded, injection-molded, unoriented crystallized specimens and capillary rheometer efflux strands of commercially stabilized polypropylene without nucleating agents were examined by optical microscopy and x-ray diffraction to determine the conditions for ,%form crystallization as a function of the distance from the surface and of the shear rate at commercial processing conditions. Results demonstrate that a t all "cooling conditions" AT = T,,, -Tb (defined as the melt temperature T , minus the bath temperature Tb) effects of strain flow initiate nucleation of &form crystals. The shear rate is demonstrated to be important for &form crystallization. A critical average threshold value for the shear rate of approximately 3 X lo2 sec-l has to be exceeded. The 0 modification is mostly connected with type-I11 spherulites and partly to row structures, and it is observed at processing conditions in oriented structures only.

This paper presents a new method for a rapid determination of the Avrami exponent n by nonisothermal thermoanalytic analysis (DSC and DTA, resp.). Contrary to conventional techniques this method can be used in the entire temperature range and therefore it is applicable to polymers crystallizing from the melt. The proposed technique is applied to injection moulded low density polyethylene (LDPE), injection moulded high density polyethylene (HDPE), unpigmented extruded polypropylene (PPunpi~m) and pigmented extruded polypropylene (PPpigm)-The resulting values for .5 " the Avraml exponents nLDPE--2.9, nHDrE--1.3, nvp --2 2 and npp ~ 2.1 derived by unlm " lm 9 .. pg]' pg . .crystalhzauon from the melt were compared with ,sothermal measurements and with results given by other authors.

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Crystallinity measurements were carried out with the help of X-ray diffraction, differential scanning calorimetry (DSC) and density measurements. The results of this study provide a better understanding of the morphology and complex thermal behaviour of polypropylene fibres. Crystallinity studies show that the crystallinity of PP fibres when measured by X-ray and DSC methods as a function of draw-ratios increases with increasing draw-ratios while the crystallinity of the same samples as derived by density measurements shows an inverse trend. These discrepancies are accounted for by the formarion of microvoids within the polypropylene fibres due to drawing. In the DSC scan two endothermic peaks were found to occur in some of the samples. This is explained by the presence of two morphological forms i. e. lamellar and fibrillar morphology in the same sample. The crystallinity values obtained from different methods are correlated with mechanical properties, such as tenacity and elongations.

It is known, that the lifetime of polyethylene pipes is essentially limited by slow crack growth (SCG). For state of the art PE materials common SCG testing methods have reached their limits with respect to extension of testing times. A comparatively new method is the Notched Ring Test (NRT) as developed by Choi et al.[1] Pipe rings notched at the inner wall are used. The test is carried out in 80 °C water under constant bending load. The arrangement of the notch at the inner wall reduces testing times using the residual stress of extruded pipes. A disadvantage of this method is that there is no clearly defined failure time because SCG takes place between two phases of creeping. The output of this test is an “on‐set slow cracking time” (crack initiation), obtained by analysis of the displacement curve. In this work it has been shown that the NRT method yields to brittle fracture within acceptable time frames.[2] Methods for data analysis are presented. This test could be very useful applied in research and development for resin evaluation and as a tool in quality control in pipe production for evaluating the process conditions.