2.4.2 Measurements
2.4.2.1 Oil Viscosity
A Haake rheometer model RS75 with a cone diameter of 60 mm and 2° angle was used to measure the viscosity of single oils and their mixtures at room temperature. The flow curve was obtained as a function of stress from 0.1 Pa to 1000 Pa.
2.4.2.2 Oil Polarity by Relative Permittivity
Oil polarity as relative permittivity can be measured by an LCR meter by Agilent Technologies [35]. Permittivity ε is a measure of the polarizability of a solvent (responsiveness of a localized charge distribution (dipole) to an external electric field), a property which is related to the polarity of the solvent molecules.
Permittivity is defined as the ratio D/E where D is the electric flux density and E is the electric field strength. The permittivity is specified in Farads per meter (F/m). It can also be defined as a dimensionless relative permittivity, or dielectric constant, normalized to the absolute vacuum permittivity ε0 = 8.85419 x10-12 F/m.
2.4.2.3 Hardness of Lipsticks
A simple lipstick containing polyethylene wax and oil was prepared by heating the mixture above 95°C until it melted completely. Then the hot mixture was poured into a lipstick mold and cooled down in a refrigerator for 20 min. The lipstick was molded and placed in a 12mm diameter bullet shape container. The lipstick was then stored at 20°C for 24 hours before measuring its hardness.
The hardness of the lipstick samples was measured using a DFGS2 dynamometer (Indelco-Chatillon). The hardness corresponds to the maximal shear force exerted by a rigid tungsten wire of diameter 250 μm, advancing at a speed of 100 mm/min.
The technique above is normally described as the “butter- cutting wire” method. The hardness value from this method is expressed in grams as the shear force required to cut a lipstick under the above conditions.
2.4.2.4 Amount and Thickness of Lipstick Deposit on Bioskin
Simple lipstick containing a single oil with polyethene wax was applied twice on a flat-type Bioskin from Beaulax Co., Ltd, Japan. The application amount of lipstick on the Bioskin was determined from the weight difference after and before application. Then, the thickness of lipstick deposit (film thickness) can be calculated from the values of application area, weight of deposit and density of lipstick.
2.4.2.5 Wax Crystallization Study
DSC was used to study the crystallization and melting behaviors of wax and wax-oil systems. The samples were heated from -30 to 140°C at a rate of 5°C/ min. After holding at 140°C for 5min, the samples were allowed to crystallize upon cooling at 5°C/min from the melt of 140°C to - 30°C. The samples were heated again from -30°C to 140°C at the same rate as cooling. Figure 2.3 shows the cooling and heating cycles for an oil-wax gel system. The parameters such as the crystallization temperature, Tc, melting temperature Tm and the enthalpies of crystallization ΔHc (J/g) and melting ΔHm (J/g) of the wax – oil systems were obtained from DSC thermograms. In addition, the starting crystallization temperature Tc,i and final melting temperature Tm,f are used in the analysis. The relative crystallinity of the samples during cooling or heating process, Xc (%), can be calculated using enthalpy of crystallization or enthalpy of melting value of 100 % crystalline polyethylene wax (PE).
The relative crystallinity Xc (%) from cooling or from melting is calculated via either equation:
Figure 2.3 DSC thermogram of an oil-wax gel system.
Figure 2.4 Lipstick sample preparation for SEM investigation.
where ΔHc0 and ΔHm0 are the enthalpies of crystallization and melting for 100% PE wax, respectively and w is the weight fraction of the PE wax in the oil-wax system.
2.4.2.6 Morphology of Wax Structure by SEM
The morphology of the wax-oil systems was studied using a HITACHI Ultra-High Resolution Scanning Electron Microscope model S-4800 at an accelerating voltage of 5 kV.
For SEM sample preparation, the procedure for extracting oils out of the wax was followed as reported by Yoshida and coworkers [16] or by Miyazaki and Marangoni [19]. In our study, a part of lipstick was used and its oils were extracted using ethanol, acetone and tetrahydrofuran (THF). It was observed that THF was the best solvent for extracting the oils without damaging the samples for SEM experiment. The lipstick sample preparation is shown in Figure 2.4.
2.5 Results and Discussion
2.5.1 Factors Affecting Lipstick Structure: Oil Viscosity
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
