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To test the effect of polarity on the crystallization of PE wax in oils, a series of ester oils with various viscosities and polarities were selected and are shown in Table 2.6. The oil polarity was measured in terms of relative permittivity (RP) for ester oils with various chemical structures. For comparison, hydrogenated polyisobutene (HPIB) a non-polar oil with low polarity was included in the data analysis. Oils with high RP are more polar than oils with low RP. For example, tri-decyl trimellitate with RP of 4.11 is more polar than isononyl isononanoate with RP of 3.29.

At a fixed 15% PE wax, the oil-wax lipstick hardness increases with increasing oil polarity (relative permittivity) (Figure 2.12). However, it is observed that the hardness of PE wax in HPIB with low RP of 2.25 and in polyglycerin-2-tri-isostearate with high RP of 3.35 does not follow the trend. This phenomenon might be due to the contribution of the high viscosity from these two oils, as both HPIB and polyglycerin-2-tri-isostearate oils have viscosities of 8.9 x10⁴ mPa.s and 2.4x10⁴ mPa.s, respectively.

Figure 2.12 Effect of oil relative permittivity on the hardness of wax-oil lipstick containing 15% PE wax.

DSC thermograms of crystallization and melting of 15% PE wax in various polar oils are shown in Figure 2.13. It is observed that by increasing RP or polarity of oils, both the starting crystallization T_c,i and final melting temperatures T_m,f of PE wax do not increase linearly as expected. This might be due to the contribution of the oil chemical structure besides the oil polarity. The melting peak temperature T_m of wax crystals and the relative crystallinity X_c determined from the wax melting process are observed to increase with increasing oil polarity as shown in Figures 2.14-2.15. However, it is not true for high viscosity ester polyglycerin-2-tri-isostearate as discussed above, which has the highest relative crystallinity and high T_m even its RP is lower compared to other ester oils. In addition, the wax in low polarity HPIB has similar crystallinity to wax in high polarity ester.

Figure 2.13 DSC thermograms of 15% PE wax in polar oils (esters) during (a) cooling and (b) heating processes.

Therefore, the oil viscosity might play a more important role in the crystallization of PE wax than the oil polarity in the simple oil-wax lipstick. As a result, the T_m and hardness of the wax-oil lipsticks are better correlated with viscosity of oils than with oil polarity as shown in Figures 2.16-2.17. In addition, the hardness of lipstick structure also corresponds with the relative crystallinity X_c determined from the melting of the PE wax in ester oils (Figure 2.18).

Figure 2.14 Effect of relative permittivity (polarity) on the relative crystallinity X_c (%) of lipstick containing 15% PE wax in oils (red circles = esters, blue circles = HPIB).

Figure 2.15 Effect of relative permittivity (polarity) on the wax melting peak T_m of lipstick containing 15% PE wax in oils (red circles = esters, blue circles = HPIB).

The morphology of the wax crystals from the polar oil-wax lipsticks studied by SEM is shown in Figure 2.19. The oil polarity has some effect on the crystal structure of PE wax, resulting in more compact structure when oil polarity increases. In a low viscosity and low polarity isononanoate, the wax crystal is the largest as observed for PE wax in the low viscosity HPIB oil from SEM in Figure 2.11. The wax crystals of sample e in Figure 2.19 have the smallest crystal size with high crystal density, contributing to the hardest lipstick structure even if the oil has lower polarity but it has the highest viscosity.

Figure 2.16 Effect of oil viscosity on the melting temperature Tm of PE wax crystals in ester oils during heating.

Figure 2.17 Effect of oil viscosity on the hardness of lipstick structure containing 15% PE wax in ester oil.

Figure 2.18 Effect of the relative crystallinity X_c (%) obtained from crystal melting on the hardness of lipstick containing 15% PE wax in ester oils.

However, the hardness of lipstick structure was found to correlate better with oil viscosity than with oil polarity. For a better understanding of the effect of oil polarity on the wax crystal structures, ester oils with similar viscosity but different polarity should be used.

Comparing the hardness of oil-wax lipsticks in polar oils, the lipsticks are harder when oil polarity increases. The results are consistent with those obtained by Imai and coworkers [17] when they investigated the effect of oil polarity on the hardness of the paraffin wax in simple oil-wax system. They found that on increasing the polarity of oil, the wax-oil gels became harder because the surfaces of crystals were rougher and wax crystals were more interconnected to strengthen the crystal networks. In addition, Li and coworkers [39] found that the presence of less polar asphaltenes (the most polar component of crude oils) in the waxy oil reduced both storage modulus and yield stress of oil-wax gel system.