New additive system for rotational sintering

image 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. Moreover, Irgastab RM 68 contains a light stabilizer that complies with the UV8 weathering standard. This worldwide standard stipulates an elongation at break of at least 50 percent of the original value after 8,000 hours of exposure to accelerated weathering. This corresponds to outdoor use in a subtropical climate for at least three years. Irgastab RM 68 has been approved in polyethylene by the American Food and Drug Administration (FDA) for indirect contact with food products.

A calculation example

A case study shows how the effect of the special additive can be expressed in concrete figures: With a conventional additive formulation in a rock-and-roll machine, operating in three shifts of eight hours, forty-four 80-liter tanks can be produced per day. The cycle time is 33 minutes. With Irgastab RM 68, the cycle time can be cut back to 27 minutes. This shortening by six minutes translates into ten additional tanks per day per machine. Extrapolated over an entire year (here 225 days), a manufacturer can thus produce about 2,300 extra tanks per machine. Moreover, the final product has a considerably lower yellowness index than products made using the conventional approach and, on top of it, it has better mechanical properties. The results will vary for each individual manufacturer as a function of the type of machine, the properties of the plastic, the geometry of the parts and the wall thickness. For this reason, BASF has also developed a program that allows the savings for the customers to be calculated on an individual basis.

Niche process on the rise

In comparison to other methods that process thermoplastics, such as blow molding or injection molding, it is true that rotational sintering is a niche process. Only about one percent of all plastics in Europe is processed by means of rotational sintering. In 2008, Irgastab RM 68 was introduced onto the American market – today still the main market for rotational sintering. But in Europe as well, this method now has become an alternative to blow-molding in recent years, especially for applications in the building sector, for toys and sports equipment as well as in the automotive and machine construction. The reason for this can be ascribed to technological improvements in the process itself. The key aspect, though, is that rotational sintering can achieve decisive cost advantages in comparison to the technically very demanding blow molding method, particularly for small production runs of large blow-molded parts. Thanks to the savings in terms of time and energy, but also due to the qualitative improvement of the final product, Irgastab RM 68 helps to optimize the process and thus contributes to its competitiveness in the market.

Presentation by Andreas Thürmer, Head of Technology and Product Management – Additive Blends for Plastics, BASF Schweiz AG, Basel, Switzerland

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