The Effect of Alkane Rotator Phases on Needle Penetration

A binary n-alkane blend (nC22+nC24) with a rotator phase stable over a wide temperature range is characterized with X-ray scattering, differential scanning calorimetry (DSC), and the ASTM needle penetration test method, which is the commonly practiced industrial specification method used to quantify the softness of wax. For wax-like solids, penetration is commonly associated with the amount of non-crystallizing liquid entrained between the alkane crystallites. Our model system has no non-crystallizable material but rather fully crystallizes first into the rotator phases, which are 3D crystals but softer than the lower-temperature non-rotator crystal phase. The penetration was measured as a function of temperature and was found to increase strongly at higher temperatures through the rotator phases, prior to the beginning of melting, where there is no entrained liquid. This shows clearly that, in addition to the entrained oil content when present, the stability of the rotator phases is an important mechanism for increased penetration and thus reduced hardness of wax.