Rotational sintering is a process that is suitable for shaping large hollow articles made of plastic, especially polyethylene (PE). These include all kinds of tanks, transportation containers and recreational articles such as kayaks and toys. The advantages of this production process include, in particular, the wide array of design possibilities and the low investment costs. Nevertheless, it entails some challenges.
Relatively long processing times and high oven temperatures place extraordinary demands on the stability of the plastic. Moreover, the process is very energy-intensive. Irgastab® RM 68 is an additive system made by BASF specifically for plastics that are processed by means of rotational sintering. It optimizes the sintering process, improves the quality of the final product and helps to save energy.
The patented combination of various stabilizers optimizes the property profile of the plastic specifically for processing by means of rotational sintering. Here, Irgastab RM 68 primarily promotes the melting behavior of the polymer during the shaping process. This allows the processing times to be shortened and the oven temperatures to be lowered, resulting in a wider processing window – an important aspect since rotational sintering often involves the use of different molds at the same time in one machine. If the residence time in the oven is the same, there is a risk that the polyethylene will not melt sufficiently in large molds while, in contrast, it will already have degraded in the small molds. In other words, the wider the processing window, the more robust the production process and thus the better the quality of the final product.
The effect of this special additive can be evaluated on the basis of the so-called Peak Internal Air Temperature (PIAT) while the process is being carried out. The PIAT indicates the air temperature in the interior of the blow mold during processing and makes it possible to follow the melting and cooling behavior of the polymer during the processing cycle.
At an oven temperature of 288 degrees Celsius [550.4 degrees Fahrenheit], the PIAT for optimally melting a three millimeter-thick test specimen made of polyethylene and containing conventional stabilizers is 233 degrees Celsius [451.4 degrees Fahrenheit]. With Irgastab RM 68, the material already reaches its optimum melting range at a PIAT of 210 degrees Celsius [410 degrees Fahrenheit]. This is why the test specimen also needs an altogether shorter cycle time in order to achieve an ideal production result. In this manner, energy savings of up to 16 percent and lower costs can be attained in comparison to the standard approaches and, at the same time, the productivity is greater.
Shorter processing times can also account for less degradation of other additives in the plastic, thus increasing the service life of the plastic part. But other quality features of the final product are also positively affected by the additive system. For instance, the inherent discoloration of the product during processing is reduced and the color fastness upon exposure to nitrogen oxide is increased. The better stabilization of the polymer chains against degradation improves the mechanical properties, especially the impact resistance of the plastic.
