Blake and Marangoni [30] reported that the needle-like morphology was observed for waxes with homogeneous compositions such as beeswax (BW), sunflower wax (SFW) and rice bran wax (RBW). Carnauba (CRW) and candelilla (CLW) waxes with heterogeneous composition were used to develop a microplatelet morphology. The DSC and XRD studies reveal that wax morphology is strongly dependent on wax composition. A wax with a chemical homogeneous composition forms a lamellar structure which organizes into needle-like crystal. A wax with a chemical heterogeneous composition develops a less ordered and smaller crystal structure.
2.3.3 Factors Affecting Oil-Wax Structures: Oil Viscosity
De Clermont-Gallerande et al. [33] reported the roles of oil viscosity and wax amount in the lipstick formulation on the mechanical properties of the lipsticks. By varying the oil viscosity or wax amount or wax type, they could control the hardness and the deposit of the lipstick which affect the sensorial perception. For example, the use of a highly viscous oil in the synthetic wax - polyethylene - polypropylene/ethylene copolymer (Tm = 820 C) increases both the breaking resistance and the thickness of lipstick compared to low viscosity oil. Using DSC, Abidh and coworkers also confirmed the strong impact of the oil viscosity on the wax crystal network which influenced the mechanical and sensorial properties of model lipsticks in a recent publication [34].
2.3.4 Factors Affecting Oil-Wax Structures: Cooling Rate
The cooling rate will affect the nucleation, crystal growth and solidification of oil-wax systems. With high cooling rate, wax crystals are smaller and have high crystal density than at low cooling. At low cooling rate, the crystal growth rate is dominant over nucleation, resulting into larger crystals.
The crystal growth from rice bran wax in hexane (non-polar oil) was investigated by using a laser diffraction technique. Ghosh and Bandyopadhyay [31] found that a high cooling rate and low temperature induced formation of a large number of small nuclei and maximal crystal size was obtained in the temperature range of 10°C-15°C.
Hwang and coworkers showed that with an increase of cooling rate from 0.1°C/min to 4°C/min, the crystal sizes of 4% sunflower wax in soybean oil reduced and the firmness of the oil-wax gels increased due to high density of small crystal formation [29].
Therefore, the hardness or firmness of oil-wax system can be modulated by cooling rate at a fixed wax concentration. A fast cooling rate will have a large impact on the hardness of the oil-wax system when a high concentration of wax is used.
2.4 Study On Model Oil-Wax System Containing Polyethylene Wax
To modulate the hardness of the solid lipstick, this study investigates the factors that might affect the hardness of simple oil-wax systems such as viscosity and polarity of various cosmetic oils with polyethylene wax. The crystallization and structures of the low molecular weight PE wax in such systems were investigated by DSC and SEM. Then the correlation between the hardness of the lipstick and its deposit amount on the bioskin is discussed.
2.4.1 Materials
Polyethylene wax with low Mw of 500 and Tm= 83°C- 90°C was used as the only wax in the study of lipsticks. The common esters and hydrocarbon oils used in lipstick formulations were selected with various viscosities and polarities (in terms of permittivity) (Table 2.6). PE waxes in a single oil or in binary oil blends of hydrogenated polyisobutene (HPIB) with low and high viscosities (ɳ=14.8 mPa.s and 89960 mPa.s) were prepared by melting the waxes in oils at high temperature.
Table 2.6 Some selective non-polar HPIB and polar oils with low and high viscosities.
| Oils | Viscosity η (mPa.s) | Relative permittivity |
| Hydrogenated polyisobutene | 15 | 2.1 |
| Hydrogenated polyisobutene | 89960 | 2.3 |
| Isononyl isononanoate | 9 | 3.29 |
| Neopentyl glycol dicaprate | 19 | 3.41 |
| Polyglyceryl-2 tri-isostearate | 290 | 3.7 |
| Diisostearyl malate | 1860 | 3.84 |
| Polyglyceryl-2 tri-isostearate | 24200 | 3.35 |
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