B. Terselius

Oxidation profiles of polyamide 6,6 studied by imaging chemiluminescence and FTIR

Abstract Injection moulded test specimens of polyamide 6,6 have been subjected to ageing in ambient atmosphere at temperatures ranging from 100 to 130 °C for different times. Skin-core oxidation was observed and evaluated by infrared spectroscopy in terms of carbonyl index depth profiles and by imaging chemiluminescence as peroxide depth profiles. The oxidation depth profiles determined by the two techniques showed good agreement with respect to ageing time and temperature. The oxidation depth was shown to decrease with increasing ageing temperature. The results clearly indicate a diffusioncontrolled oxidation process.

Gelation of rigid PVC-pipes

Abstract Balancing the properties of rigid PVC products requires an optimization of the gelation level. In this work different methods have been used to characterize the gelation level of a set of PVC pipes extruded at different temperatures. From capillary rheometry measurements at 145°C using a flat die, it is concluded that the gelation level should be related not only to one specific compound but to one specific reference process. The die swell was increased with the gelation level in about the same way as the flow pressure. The extrusion temperatures of the pipes ranged from 180 to 204°C and corresponded to gelation levels between 10 and 75% based on the roll mill reference curve. Methylene chloride swelling was small at 50% gelation level and negligible above 70%. Different impact tests showed a broad maximum at about 50%. The falling weight method “B 50,” however, showed a remarkably distinct maximum. Physical aging by slow cooling from above Tg of the pipe compound did not influence the process hi...

A new method for the detection of thermal oxidation in polyethylene pipes

Abstract Dumb-bell shaped specimens containing the inner wall surface of the pipe cut from 12 polyethylene pipes of different origin were subjected to constant uniaxial tensile loads at 313 K in air. The brittleness of the inner wall surface layer of the thermally oxidized pipes manifested itself in a shift of the unstable/stable necking transition to lower stresses and longer times. With the aid of optical and scanning electron microscopy, the brittleness of the inner wall surface layer of the oxidized pipes was demonstrated in distinctive surface cracks, which were the dominant feature of the drawn samples of the oxidized pipes. No such surface cracks were observed in the non-oxidized samples. The surface crack patterns were characterized in terms of crack frequency (longitudinal and transverse), width of fragments of cracked top layer (lB), plastic strain of the fragments of the cracked top layer (epl) and thickness of the cracked top layer (H). Correlations were found that can be interpreted according to basic principles of fracture mechanics. Through knowledge of, for example, epl and lB the values of the other variables can be predicted. The thickness of the cracked top layer corrected for the reduction in thickness due to plastic deformation is approximately equal to the thickness of the oxidized layer as determined by polarized microscopy. Drawing in an Instron Tensile Testing Machine at an elongation rate of 10 mm/min at 298 K also revealed the distinctive surface cracks in the oxidized samples. On the basis of these results, a new method for the detection of thermal oxidation in polyethylene pipes is proposed.

Influence of microtomy on measurements of orientation

Abstract The correct measurement of orientation and crystallinity on thin sections microtomed from larger specimens requires a knowledge of the influence of sectioning on these properties. In this work, the measuring technique used is FTIR, using dichroism to determine the orientation and regular FTIR to determine the degree of crystallinity. This paper shows that the sectioning influences the measured orientation of the sections in the case of both amorphous polycarbonate and crystalline polypropylene, while the crystallinity seems to be unaffected in the case of the unoriented samples.

A survey of methods for the detection of thermal oxidation in high-density polyethylene pipes

Abstract Seven methods for the detection of thermal oxidation of the inner wall surface of high-density polyethylene (HDPE) pipes are presented. The methods presented include infrared spectroscopy, polarized light microscopy, differential scanning calorimetry, scanning electron microscopy, gloss measurements and uniaxial creep tests. These tests have been developed on the basis of earlier reported data for a large number of PE pipes. The tests are compared with each other and with the internal pressurizing test with respect to reliability of results, the time taken to determine whether or not a pipe is oxidized, the experimental difficulties, costs, etc.

Effects of Processing on the Structure and Properties of PVC

This work includes an extensive study of the effect of the gelation or fusion stage of processing on the ultimate mechanical properties of PVC pipes by variation of the mass temperature at extrusion. Moreover, the influence of gelation on the supermolecular structure and on some physical properties was studied.

Structure and Fracture of Thermally Oxidized Pipes of High-Density Polyethylene

The structure and fracture properties of high-density poly ethylene pipes, thermally oxidized during processing, were studied.

The Influence of Crystalline Structure on the Necking-Fracture Behaviour of Polyethylene

An anomaly in the necking and fracture behaviour of polyethylene under constant uniaxial tensile loading has been reported in several papers1–3. At a certain stress level, the almost instantaneous fracture of the neck formed at high loads was replaced by the formation of a neck that resisted fracture for a considerable time. This marked transition was observed for high density polyethylenes of comparatively high molecular weight. A hypothesis has been proposed to explain the appearance of the marked transition: 1. At high strain rates, large stresses concentrate to the taut tie chains between the crystallites, favouring fracture processes. At lower strain rates, the molecules rearrange in a way that distributes the local stresses more evenly over the structure which then deforms further rather than fracturing. 2. Owing to the high concentration of the chains in the high density, high molecular weight polyethylenes, the transfer of stresses and deformations between adjacent crystallites is very efficient. Therefore the structure has a good resistance towards fracture and the deformation of different lamellar regions occurs cooperatively. 3. The marked transition is a result of both (a) the rapid formation of high strength fibrillar structure, which is partly due to reasons given in item 2 of this hypothesis, and (b) the influence on the strength of the strain rate in accordance with item 1 of the hypothesis. At the marked transition, the maximum in the local strain rate is about the highest tolerable for the actual structure without macroscopic fracture. When an element of the specimen passes the maximum in local strain rate the probability for fracture of this element decreases due to the decreasing strain rate and to the continuous formation of a high strength fibrillar structure. In addition, e.g. the rise in local temperature in the neck caused by dissipation of deformation energy,may affect the necking and fracture behaviour.