M. C. Cramez

Effect of nucleating agents and cooling rate on the microstructure and properties of a rotational moulding grade of polypropylene

Rotational moulding consists of coating the inside surface of a metal mould with a layer of plastic by rotating the mould, firstly in an oven and then in a cooling bay to induce solidification to the desired part shape. As the rotational speeds are slow (typically about 10 rev/min), the resulting hollow articles are practically stress free. The primary material used for rotationally moulded parts is polyethylene but there is an increasing interest in using polypropylene to provide stiffer, higher temperature products. Unfortunately the slow cooling combined with the slow crystallisation rate of polypropylene results in brittle mouldings with coarse spherulites. Since the inner surface of the plastic is in contact with air during moulding, degradation is also likely to occur. In order to improve the mechanical properties of the rotationally moulded polypropylene, α and β nucleating additives were added. The effect of using faster cooling rates was also studied. It was found that heterogeneous nucleation, both of β and α spherulites, did not improve the ductility of the samples. However, when fast cooling was used, the impact strength of the polypropylene improved markedly, independent of the presence of nucleating additives. In the rotationally moulded polypropylene parts, the fast cooling could only be applied to the outer surface of the mould, which led to asymmetric cooling. This resulted in severe warpage, and uneven morphology. This problem should be overcome by using fast cooling on both the inside and outside surfaces of the plastic.

Structure-properties relationships in rotationally moulded polyethylene

The relationship between structure and properties of rotationally moulded polyethylene was studied using three different contents of antioxidant — 0.04% (standard) 0.1% and 1% — and two different mould atmospheres — air and nitrogen. The mechanical behaviour of the moulded parts was interpreted in terms of the structure of the polymer and the level of degradation at the inner surface of the mouldings. The degree of degradation was assessed using Fourier transform-infrared spectroscopy, fluorescence microscopy and melt-flow index measurements. The microstructure was observed using polarized light microscopy and scanning electron microscopy. The mechanical strength was evaluated by impact testing using an instrumented drop weight machine. The results showed that as the maximum temperature of the gas inside the moulding increases, the impact strength also increases, reaching a maximum at about 225 °C. This build up in impact strength is related to the improved sintering of the plastic powder/melt with time and temperature. However, as is well known in the industry, the moulding conditions needed to achieve optimum properties are critical because a small amount of overheating causes the impact strength, for example, to decrease dramatically. This decrease is due to the degradation that occurs at the inner surface of the samples. This occurs because the surface is in contact with oxygen during processing, leading to oxidation reactions in the material, predominantly resulting in cross-linking. The cross-linked material is responsible for the low impact strength and for the brittle behaviour of samples moulded at higher temperatures. The morphology of these samples is not characterized by the polyethylene spherulitic texture, observed across the thickness of the samples moulded at lower temperatures. The morphology at the inside surface was modified and gave rise to a thin layer of very small and imperfect spherulites, which disappear when the heating is too severe. Next to it is a columnar-type structure made of bundles of parallel fibrils. The increase of antioxidant content and the use of a nitrogen atmosphere caused a delay of the degradation process, but did not prevent it. No significant mechanical strength improvements were observed in these conditions, but a broader processing window was available.

Optimisation of rotational moulding of polyethylene by predicting antioxidant consumption

Rotational moulding is used to manufacture hollow plastic products. The process offers many advantages to the designer, but it is hampered by a strong dependence on trial and error methods to achieve good part quality at economic production rates. During rotational moulding, the polymer is subjected to relatively high temperatures for long periods of time in the presence of air. This can lead to degradation of the polymer at the inner free surface of the moulded article, with consequent deterioration of the mechanical properties of the part. The processing conditions that lead to degradation vary with factors that affect the heating rate, such as the type of mould used. In this work a method is proposed to predict the onset of degradation, on the basis that this occurs when the concentration of anti-antioxidant in the polymer reaches zero. Good agreement between the experimental and predicted optimum processing temperature was obtained for two grades of polyethylene stabilised with two different antioxidant systems. Using the method described, it is now possible to identify the best rotational moulding conditions for a particular polymer so that more efficient cost-effective parts can be produced.

ROTATIONAL MOLDING OF POLYOLEFINS: PROCESSING, MORPHOLOGY, AND PROPERTIES

In the rotational molding of semicrystalline polymers, the slow heating and cooling rates and an almost absence of shear stresses lead to coarse spherulitic morphologies free from molecular orientation. This molding process, which is suitable for short runs of large parts, is not favorable for the dispersion of additives like pigments. This article reviews the type of morphologies that develop during the rotational molding of polyethylene (PE) and polypropylene (PP) when mixed with nucleating pigments or with different amounts of antioxidants. Over a range of typical processing conditions, it was observed that the morphology is affected by the processing temperature. Especially at the inner surface that is in contact with air, the sensitivity to temperature is higher, and degradation is more likely to occur. In the case of PE, the degradation is revealed in the morphology by the suppression of crystallization, whereas for PP, a change in the appearance and the level of birefringence of the spherulites occurs. Degradation may be delayed by increasing the antioxidant content or using an inert atmosphere, but without significant mechanical strength improvement. The morphology is affected by the way the nucleating pigments are incorporated during the mixing process. For example, the poor mixing capability of turboblending leads to the pigment concentrating around the polymer particles and to development of transcrystalline textures. Conversely, the more effective extrusion compounding allows the pigment to disperse and distribute better and leads to the reduction of the spherulite size. In the case of PP, which is a candidate for rotational molding, an improvement of the impact strength is achieved only if the finer morphology is associated with a significantly lower degree of crystallinity.

Prediction of Spherulite Size in Rotationally Molded Polypropylene

Rotational molding is used to manufacture hollow plastic parts. It is characterized by relatively slow cooling rates, which leads to large spherulites and brittleness in rotomolded polypropylene parts. Using both theoretical and experimental methods, this article assesses the factors that control spherulite size so that the properties of rotationally molded polypropylene parts can be improved. The approach taken is to predict the average density of the nuclei of isothermally crystallized polypropylene as a function of the crystallization temperature, using data on the half-time of crystallization (determined by differential scanning calorimetry) and the spherulite growth rate (measured by optical microscopy). The prediction method is then extended to nonisothermal quiescent crystallization, such as occurs in rotational molding, by determining the temperature corresponding to half of the phase change and its relationship with the cooling rate. To establish the average true sample temperature on cooling, experimental data are corrected for the temperature calibration at a particular cooling rate, the thermal resistance of the sample, and the release of the heat of crystallization. The surface nuclei density of polypropylene specimens, as crystallized isothermally and nonisothermally in differential scanning calorimetry, and also as processed by rotational molding, was determined by optical microscopy and converted to volume density using the Voronoy relationship. A good agreement was found to exist between the experimental results and the predictions.

Effect of Pigmentation on the Microstructure and Properties of Rotationally Molded Polyethylene

Rotational molding of plastics has experienced growth rates of about 12% per annum over the past decade. As a result, ever more demands are being placed on the quality of the moldings in terms of dimensional control and mechanical properties. With most molding methods for plastics, the use of pigments can have a significant effect on the quality of the product. This is particularly true for rotational molding because there are no stresses to assist with dispersion of the pigment, and the slow cooling rates encourage classic spherulite formation. This paper investigates the use of nucleating and non-nucleating pigments in a rotational molding grade of polyethylene. We demonstrate that the amount of work done on the plastic prior to molding affects the microstructure and the mechanical properties of the end product, often in a positive manner. Turbo-blending of pigments is shown to be problematic, particularly if the pigment is of the nucleating type. The amount of pigment used has little effect on strength but reduces toughness dramatically.

Nanostructure Evolution during Uni-Axial Deformation of PET – A WAXS and SAXS Study Using Synchrotron Radiation

In this work, the structural evolution and damage of PET during stretching is assessed by wide- and small-angle X-ray scattering (respectively, WAXS and SAXS) experiments in specimens pre-deformed at different strain levels (ex-situ characterization). Injection moulded PET rectangular tensile specimens were stretched (at 2 mm/min) into the plastic domain in a universal test machine at different strain levels at room temperature. The structure of the central zone of the deformed specimens was then characterized by WAXS and SAXS experiments using an X-ray synchrotron source. PET was initially (before stretching) amorphous. A strong molecular orientation in the stretching direction is quickly developed for the initial plastic deformation levels, evidenced by strong equatorial WAXS reflections. This orientation rapidly levels off, remaining constant during further stretching. The WAXS patterns are accompanied with no reflections on SAXS, evidencing a local ordering phenomenon, typical of nematic liquid-crystalline structures. The SAXS patterns evidence the occurrence of some voiding in the cold drawing regime just after yielding. These anisotropic voids are oriented perpendicular to the stretching direction, as in a craze-like structure. The void size drastically increases at the onset of the strong strain hardening behaviour.

Relationship Between the Microstructure and the Properties of Rotationally Molded Plastics

ABSTRACT The rotational molding process has characteristic features that make the microstructure of the molded plastic articles unique. The fact that the rotation speeds are slow (typically less than 10 rpm) results in low shear which promotes the development of textures that are free from orientation, but prevents the dispersion additives like pigments. Also, if the heating of the polymer is too severe, degradation may occur at the inner free surface, causing the spherulites that grow freely at that surface to be replaced by a non-spherulitic or a transcrystalline texture, depending on the extent of degradation. The polymer microstructure in the bulk, and also at the inner surface layer has a major influence on the mechanical properties of the molded material. In this paper, work done on polyethylene and polypropylene rotational moldings will be presented to illustrate the influence of a number of factors: molding temperature, grinding and mixing conditions, type of pigment, anti-oxidant level, mold material and inner atmosphere. Polarized light, common light and DIC microscopy were used to study the microstructure of the moldings. The occurrence of degradation was assessed by FTIR spectroscopy and fluorescence microscopy. The rheological properties of the material after processing were determined with a cone and plate rheometer. The mechanical properties were measured by instrumented falling weight impact and tensile tests. As a result of this extensive experimental program, it has been found that in rotational molding, polyethylene and polypropylene polymers show different degradation behavior. While in polypropylene the thermo-oxidative degradation causes mainly chain scission, in polyethylene crosslinking dominates. The use of increased amounts of antioxidant in the polymer, or the use of an inert atmosphere, delays the degradation but does not prevent it. It has also been observed that the thermo-mechanical effects caused by the mixing processes commonly used to add pigments increase the mechanical properties of polyethylene products. Finally, the nucleating activity of the pigment combined with the mixing process, has a major effect on the microstructure and on the mechanical properties of the final products.