Soft TPS compound based on Kraton MD1648 SEBS polymer were recently developed for micro pellets used in rotational moulding applications. Kraton said that the use of this high flow SEBS polymer for industrial compound applications demonstrates that it is possible to obtain a stable, repeatable industrial micro pelletisation compounding process under commercial run rates for TPS micro pellets. The micro pellet technology is a proven alternative to cryogenic milled powder solutions. With the ongoing polymer and micro pelletisation technology development, smaller particle size distributions are achievable.
This development also demonstrates the technical feasibility of producing a high flow Kraton MD1648 polymer-based TPS compound on an industrial compounding line as a micro pellet using commercially-available underwater pelletisation technology. The D50 particle size distribution during this development was less than 600 μm. The smallest particle size distribution was obtained at low extruder throughput rates.
The process conditions to rotational mould TPS micro pellets using a standard box mould were established using the design-of-experience approach. The low viscosity rheology allowed processing at modest temperatures (200-220 °C peak internal air temperature). Although the pellets were relatively large in size compared to powder, the mould design limitations seemed minimal. The TPS micro pellets could successfully be used on commercial rotational moulding equipment under standard processing conditions. Symmetric seat cushions and box samples with a high surface quality were produced from this process.
Micro pellets produced during compounding is an attractive alternative to cryogenic milled powder, which has a wide particle size distribution range that can negatively impact powder flow and caking behavior. Micro pellets’ particle size distribution is very sharp without fine particles, which guarantee excellent solid flow and substantially reduce caking behavior. Micro pellet production can be achieved in a single production step – cryogenic milled powder is usually a three-step production process – so economic and supply chain complexity is substantially reduced.