Effect of Comonomer on Crack Resistance

Having review Grade Selection and the effect of Melt Flow Index, another factor to consider in deciding on a grade for rotational moulding of water tanks is the effect of comoner on crack resistance as the most important function of a polytank is containment!  That’s why it’s very important for a tank to be resistant to cracking.

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 into 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 created by the incorporation of butene. As a result, the chain deviation caused by hexene is significantly greater than that caused by the incorporation of butene and results in the hexene LLDPE exhibiting a crack resistance approximately 10 times better than the butene LLDPE. The incorporation of octene into the polymer chain creates only a slightly larger deviation than that created by the incorporation of hexene and the change in crack resistance is less significant.

Alkatuff LLDPE rotational moulding grades utilize hexene comonomer resulting in excellent crack resistance.

The next blog post will explore the effects of weather resistance in polyethylene and how added UV should affect the decision for which moulding grade to choose.