A study on shrinkage and warpage of rotational moulded polyethylene

Warpage and poor dimensional stability of rotomoulded products are two of the main obstacles to the use of this technique in the production of engineering parts. The knowledge of the effect of the processing conditions on the shrinkage of rotomoulded parts will allow overcoming some of the restrictions of this process. In the present work the influence of the processing conditions on the development of shrinkage and warpage of rotomoulded parts was studied. The moulding of the parts was performed using a rotational moulding machine build at the University of Minho. The shrinkage and the warpage of the moulded parts were assessed using 3D MMC (3D measuring Machine Control) equipment, and understanding the microstructural development

Effect of weathering conditions on the degradation behaviour of ABS

Fundação para a Ciência e a Tecnologia (FCT) - Bolsa de Doutoramento SFRH/BDE/15657/2007Poliversal – Plásticos e Tecnologia, S.A

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

Outdoor and accelerated weathering of acrylonitrile-butadiene-styrene : a correlation study

The influence of outdoor and accelerated weathering conditions on the photo-oxidative degradation of stabilized acrylonitrile-butadiene-styrene (ABS) was investigated, aiming to establish a correlation between both weathering types. ABS samples containing combinations of light stabilizers (ultraviolet absorbers e UVA and hindered amine light stabilizers e HALS) and antioxidants were prepared by melt compounding, exposed to different weathering conditions and characterized by infrared spectroscopy and quasi-static measure ments. The results obtained by infrared spectroscopy and quasi-static measurements are in good agreement, showing that the Q-Sun Xe-3-Hs resembles the outdoor weathering phenomena. Applying a shift factor of 0.85 MJ m 2 to the abscissa axis, it was found that the exposure of stabilized ABS samples to ca. 1260 h in Q-Sun Xe-3-Hs induces changes in its mechanical performance, which corresponds to one year of outdoor exposure at LisbonThe authors would like to thank the Portuguese Foundation for the Science and Technology (FCT) for the PhD grant SFRH/BDE/15657/2007, PEst-C/CTM/LA0025/2011, CQ/UM- PEst-C/QUI/UI0686/2011 and Poliversal - Plasticos e Tecnologia, S.A

Natural and accelerated weathering of Acrylonitrile-Butadiene-Styrene (ABS)

Spectroscopic methods, UV/Vis and Fourier Transform Infrared with an attenuated total reflectance (ATR) accessory, were applied to evaluate the influence of light stabilizers on ABS samples exposed to accelerated and natural weathering conditions up to 600h and 360 days, respectively. Some important differences in the formation rate of carbonyl compounds were found when comparing accelerated to natural weathering exposure. While stabilized samples exposed to accelerated weathering reveals slower formation of carbonyl groups than unstabilized ABS, the carbonyl absorption of samples exposed to natural weathering is comparable to the unstabilized ones. The behaviour of light stabilizers during natural exposure is somewhat surprising and cannot be only associated to its stabilizing effect. The disappearance of light stabilizers only after one month of exposure could be due to molecular scission of light stabilizers with consequence volatilization and diffusion of these fragments on the surface, while under acceleration conditions seems to occur by photochemical reactions. The results obtained are interesting showing that sometimes the material behavior under real weathering conditions cannot be predicted from the results of accelerated laboratory experiments.Fundação para a Ciência e a Tecnologia (FCT) - SFRD/BD/48512/200

Optimization of the rotational moulding process for polyolefins

Hollow plastic parts can be made by a number of manufacturing methods, but only rotational moulding offers the ability to create one-piece, stress-free parts with attractive economics. However, the fundamental nature of rotational moulding is such that cycle times are long. Consequently, the plastic is subjected to relatively high temperatures, in the presence of air, for excessive periods of time. This can lead to thermal and oxidative degradation at the inner free surface of the plastic, resulting in a deterioration in the performance of the moulded part. The industry relies heavily on the experience of operators to establish the best processing conditions to avoid degradation, but this is problematic and inefficient. Unfortunately, automatic process control is difficult owing to the complex rotation of the mould. Recent developments highlighting the importance of the peak air temperature inside the mould have been an important step forward, but ever-increasing technical demands on moulders make it clear that more sophisticated process control is needed. It is known that the processing conditions that lead to degradation vary with factors that affect the heating rate, such as the type of mould used and the thickness of the end-product. In the work reported here, a method is proposed for predicting the onset of degradation, on the basis that this occurs when the concentration of antioxidant in the polymer reaches zero. Good agreement has been obtained between the experimental and predicted optimum processing temperature for polyethylenes stabilized with different antioxidant systems. A procedure is described for identifying the best rotational moulding conditions so that more efficient manufacturing methods can be achieved

Nanostructure evolution during unixial deformation of PET : a simultaneous 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.(undefined