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6 ROTATIONAL MOULDING INTRODUCTION For many years, low density polyethylene (LDPE) powders were used for the rotational moulding process. However, since the early 1990’s, linear low density polyethylenes (LLDPEs) have come to the fore in rotational moulding applications, because of their outstanding physical properties (excellent environmental stress crack resistance and impact performance) and ease of processing. Alkatuff LLDPE powders are eminently suitable for rotational moulding for these reasons. GRADE SELECTION FOR ROTATIONAL MOULDING The Alkatuff grade range is designed to provide optimimum performance in moulding and durability. The grades provide a range in flow, balanced with excellent crack resistance. Alkatuff LL711UV is a premium tank grade providing high rigidity along with excellent processability. This material is known for its proven performance and broad processing window. For smaller mouldings where rigidity is of less importance, the higher flow Alkatuff LL705UV, again with excellent crack resistance may be suitable. Both Alkatuff LL711UV and LL705UV contain premium UV and antioxidant packages and meet the requirements of the tank standard AS/NZS 4766. Both are supported with technical data to enable Finite Element Anaylsis (FEA) of designs. For parts requiring higher flow and sharpness of detail, but allowing greater flexibility, the higher flow grade Alkatuff LL710UV may be considered. Effect of Melt Flow Index and Density The end-use properties of the rotationally moulded product will depend on the density and Melt Flow Index (MFI) of the polyethylene used. Increasing the density leads to increased stiffness as well as a higher softening temperature, surface hardness and abrasion resistance. A higher MFI is desirable for ease of flow but low MFI leads to better toughness and environmental stress crack resistance. Polyethylenes with an MFI between 2 and 20 g/10min can be processed by rotational moulding, but normally grades with an MFI of approximately 3 to 5 g/10min are most suitable. The effects of changes in MFI and density on the properties of polyethylene rotational mouldings are shown in Figure 1. Figure 1: The Effects of Changes in MFI and Density on Properties of Polyethylene Rotational Mouldings Effect of Comonomer on Crack Resistance When subjected to stress over a period of time most materials commonly fail by cracking. Polyethylene may also suffer this mode of failure. It is important to understand that different grades of Polyethylene can exhibit vastly different performance with respect to crack resistance. The crack resistance should be a major consideration when selecting a grade of polyethylene for a particular application. The crack resistance of a specific grade of Linear Low Density Polyethylene is governed largely by the size of its molecules (indicated by its Melt Index) and the type and level of comonomer employed in its manufacture. Comonomers such as butene (C4), hexene (C6) and octene (C8) are reacted into the polymer chain in order to produce small side chains which in turn create a deviation, or change of angle in the molecule chain. The effect of this change of angle is somewhat akin to the effect of a knot in a piece of wood. The Polyethylene becomes more difficult to split. The size and quantity of comonomer molecules introduced in to the polymer chain determine the degree to which the crack resistance is improved. For example, the side chain created by the incorporation of hexene is twice the size of that 4 Qenos Technical GuidesPDF Image | ROTATIONAL MOULDING Guide
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