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The viscosities of selected cosmetic oils were measured at room temperature before blending them with PE wax. To verify the effect of oil viscosity on the hardness of an oil-wax system, the low and high viscosity hydrogenated polyisobutene (HPIB) oils from Table 2.6 were blended at various ratios for this study. Then simple lipsticks were prepared by dissolving PE wax at certain amount in individual oil or oil blend at high temperature. At a fixed amount of PE wax (15% by weight), the oil - wax lipstick hardness is observed to increase with increasing oil viscosity (Figure 2.5).

Figures 2.6a-b show the cooling and heating curves of the PE wax in oil-wax systems from DSC. During the cooling process, a bimodal crystallization peak is observed when PE wax is in a high viscosity oil, while a single broad peak appears when PE wax is in a low viscosity oil. The blending ratio of the low and high viscosity HPIB also affects the crystallization of PE wax, exhibiting a bimodal peak from the high viscosity oil portion (Figure 2.6a). This indicates that there are 2 different populations of wax crystal sizes formed in high viscosity HPIB and in its blends. The bimodal peaks are also observed during wax melting in high viscosity HPIB and its blends (Figure 2.6b). Furthermore, in the cooling process from the melt, the starting crystallization temperature T_c,i of PE wax shows a strong dependence on the viscosity of the oil. T_c,i starts at higher temperature for PE wax in high viscosity oil and at lower temperature for wax in lower oil viscosity. Similarly, the final or melting onset temperature of wax crystals T_m,f finishes at higher T_m for wax in high viscosity oil. There is a hysteresis between the initial crystallization and final melting temperatures of around 11 °C as observed in Figure 2.7, indicating crystal growth after annealing at low temperature.

The crystallization and melting enthalpies of 15% PE wax in various HPIB oil blend ratios were obtained from the areas under the crystallization and melting peaks in DSC. Increasing the viscosity of the oil blend increases the enthalpy of crystallization of the wax-oil system, hence the % relative crystallinity of PE wax (determined from equation (2.1)) increases (Figure 2.8). The low relative crystallinity of PE wax in low viscosity HPIB might be from a portion of wax crystals dissolved in low viscosity oil. The relative crystallinity is higher in high viscosity HPIB. In the heating cycle, the melting peak temperature T_m of wax-oil gels also shows a strong viscosity dependence as shown in Figure 2.9.

Figure 2.5 Hardness of lipstick structure (containing 15% PE wax) as a function of hydrogenated polyisobutene (HPIB) oil viscosity.

Figure 2.6 DSC thermograms of 15% PE wax in low viscosity (LV) HPIB and high viscosity (HV) HPIB and their blends during (a) cooling and (b) heating.

The % relative crystallinity X_c of PE wax in the HPIB oil during melting was calculated from equation (2.2) using the melting enthalpy ΔH_m obtained under melting peak and ΔH_m0 of pure PE (230.5 J/g). It is observed that the hardness of lipstick increases with an increase of relative crystallinity X_c (from melting) (Figure 2.10). However, two samples c and e with lower relative crystallinity in the oil blend have higher hardness. This behavior might be due to the size and shape of wax crystals developed in the oil blends and influencing the hardness of lipstick. In addition, the relative wax crystallinity in the melting cycle is observed to increase after being annealed at low temperature. Therefore, the strong dependence of lipstick hardness on oil viscosity cannot be explained by crystal density alone.

Figure 2.7 Effect of oil viscosity on the initial crystallization temperature T_c,i (blue circles) and final melting temperature T_m,f (red circles) of 15% PE wax in HPIB oils.

Figure 2.8 Effect of HPIB oil viscosity on the relative crystallinity X_c during cooling.

Figure 2.9 Effect of HPIB oil viscosity on the melting peak temperature Tm of 15% PE wax.

The SEM micrographs for the wax- HPIB oil systems reveal that the crystal size of PE wax reduces when oil viscosity increases (Figure 2.11). Low viscosity of HPIB (ɳ = 15 mPa.s) creates large wax crystals (sample a) while high viscosity HPIB (ɳ = 8.9x10⁴ mPa.s) creates small crystals (sample b). Depending on the oil blend viscosity, different wax crystal sizes and densities are observed. The sample c is wax structure crystallized in 3 to 1 ratio of low to high HPIB oil viscosity, having large crystal size and more contact points than sample a. Therefore, the sample c has harder structure compared to sample a, and even the relative crystallinity is a little lower as seen in Figure 2.10. Sample e is a wax structure crystallized in 1 to 3 ratio of low to high oil viscosity, having a smaller crystal size and higher hardness than sample d. Thus, the reduction of crystal size will increase the contact points of each crystal, enhancing the strength of lipstick [16]. For the wax crystal sample d (in the blend of 1 to 1 ratio of low and high viscosity HPIB), the crystal size is largest and open cell, which contributes to the weaker structure. Therefore, the hardness of the lipsticks is influenced by the densities, sizes, shapes and contact points of wax crystals as reported for wax-oil gels [10, 19, 29, 36, 38].

Figure 2.10 Effect of the % relative crystallinity X_c (from wax melting) on the hardness of the lipsticks containing 15% PE wax in HPIB oils.

Figure 2.11 SEM micrographs of PE wax-oil lipsticks at fixed 15% PE wax in hydrogenated polyisobutene (HPIB) with oil viscosity (a) ɳ _HPIB = 15 mPa.s, (b) ɳ _HPIB = 89,960 mPa.s, and with oil blend viscosity (c) ɳ _{HPIB blend} = 72 mPa.s, (d) ɳ _{HPIB blend} = 900 mPa.s and (e) ɳ _{HPIB blend} = 9,200 mPa.s.

From these results, the viscosity of the oil-wax system during cooling has a strong effect on the crystal nucleation and growth. When the temperature is reduced during the cooling process, the viscosity of oil-wax solution increases and crystals start to nucleate in oil and aggregate to form a gel network. In a high viscosity solution, the wax solution becomes gel faster during the cooling process, and the nucleation and growth of crystals will slow down, forming smaller wax crystals compared to the wax crystallized in a lower viscosity solution. Depending on the viscosity of the oil-wax gel at the gelling temperature, the aggregation of crystals would influence the strength of wax network and the final oil-wax lipstick structure. Therefore, the oil viscosity has a strong impact on the crystallization of PE wax, resulting in different crystal sizes, shapes and densities as observed from SEM in Figure 2.11. The dependence of the hardness of the oil-wax system on the oil viscosity was also observed and discussed by Abidh et al. [34] or by Dassanayake and coworkers when they studied the kinetics of crystallization of rice bran wax (RBW) in vegetable oils [38].