following factors:
Type of wax and concentration of wax
Polarity of oils
Viscosity of oils
Cooling rate
2.3.1 Factors Affecting Oil-Wax Structures: Wax Type
2.3.1.1 Non-Natural Waxes
To improve the structure of lipstick, Yoshida and coworkers [16] discovered that when a small amount of branched paraffin wax was added to a linear paraffin wax in ester oil, neopentyl glycol dicaprate, the hardness of the lipstick was enhanced. This behavior was from the reduction of the wax crystal size making the gel harder, but at the same time the crystal density decreased making the gel softer. To prevent the reduction of wax crystal density, they developed a branched wax, bis(polyethylenyl)-tetramethyldisiloxane, in which polyethylene (PE) units were at both ends. With the tetramethyldisiloxane at the center, it suppressed the crystal growth, but the PE end-units prevented the decrease of crystal density. Therefore, by adding this branched wax in the n-paraffin wax, the gel hardness increased dramatically. The benefit of this branched wax was to improve the hardness of the lipstick with lower amount of wax, which enhanced the gloss and gave creamy/smooth texture upon application.
To increase the hardness of oil-wax gel in a lower polarity oil, Imai and coworkers [17] blended 2 paraffin waxes with different carbon chain lengths (C30 and C32) in 1 to 1 ratio, and the gel was harder and rougher compared to the single paraffin wax gel, but still exhibited a single crystal phase as determined from X-ray diffraction (XRD). However, the blend of 2 paraffin waxes of carbon chain lengths C32 and C28 caused a macro-phase separation, resulting in a softer gel than from the blend of C30/C32 and the surface of crystals was smooth. This phenomenon was explained by the disorder of lamellar structures caused by the crystallization of 2 waxes with different carbon chain lengths as observed by SEM, small angle X-ray scattering (SAXS) and XRD measurements.
Park and Ha [18] investigated the effect of lattice structure of wax crystals on the glossiness of wax-oil systems. Adding a branched microcrystalline wax with lower Tm to ceresin wax in polyglyceryl-2-tri-isostearate oil reduced the hardness of wax-oil gels, enhancing the gloss. The reduction of wax-oil gel hardness was due to the enlarged and loose lattice structure of ceresin wax crystals in the presence of branched wax, and decreased crystal density as observed by SEM. However, when adding n-paraffin wax with lower Tm to ceresin wax, the dense lattice structure of wax crystals did not change much; hence, the hardness of wax-oil gels was slightly reduced, resulting in a low glossiness from the wax-oil gels.
Similarly, Miyazaki and Marangoni studied the structure and mechanical properties of straight-chain paraffin wax (Tm= 72°C-74°C) and polyethylene wax (Tm= 83°C-90°C) in the select oil isotridecyl isononanoate [19]. Their results showed that PE wax-oil system had a higher Young’s modulus and yield stress than the paraffin wax- oil system because the size of the PE wax cells was smaller as observed by cryo-SEM.
For a long lasting color lipstick, the crystallization of wax from the formulation containing silicone resin and volatile oils posed a challenge to create rigid structure for a solid lipstick. In the long-wear lipstick formulation, the volatile oil, isododecane, has low viscosity and is a good solvent for PE wax. In order to create a rigid solid form, high amount of PE wax is used to solidify the formulation. As a result, the attributes of wear, spreadability and sensorial perception of the solid lipstick are sacrificed because the solid lipstick is too hard to give a good deposit and smooth application. To solve this problem, the blend of high melting and low melting PE waxes is used [20]. Furthermore, adding a small amount of high melting alcohol wax to the blend of PE waxes helped to achieve the right structure of the lipstick with good deposit and maintaining long-lasting color.
Nicholas and Brooks reported the use of hyperbranched polyalphaolefin waxes (HBWs) to enhance the hardness of lipstick [21]. Increasing the molecular weight and branch length, HBW can exhibit from liquid to high melting solid wax. When blending a small amount of short branch length HBW (Tm= 41°C) with high melting point polyethylene wax in volatile oil, isododecane, the hardness of the lipstick was enhanced. The SEM showed that adding HBW to PE wax created more uniform and smaller crystal structures. They concluded that the HBW acted as a compatibilizer between isododecane and PE wax, resulting in smaller crystal sizes and enhancing both the stability and hardness of the lipstick.
Kose [22] developed a new technology to control the ultra-fine structure of lipstick in order to deliver high shine and smooth application for consumers. They controlled the interior wall of wax structures, which is known as card-house structure, by 200 times smaller than the existing wax-oil gels. By utilizing amorphous polypropylene as a thin amorphous layer between the crystalline phases, the coating of applied lipstick became more uniform and shiny on the lips.
2.3.1.2 Natural Waxes
To enhance the hardness of the low melting wax in oil, Endo and Shibata blended the high melting point alcohol (Tm= 80°C) with low melting point rice bran wax (Tm= 72°C) in macadamia nut oil [23]. The gel hardness improvement was explained by the disappearance of spherical clusters present in the gel and the creation of more uniform structure of the oil-wax blend compared to the single wax gel system as observed by SEM.
Tavernier et al. [24] investigated the crystallization behavior of high and low melting wax blend in the rice bran oil. The hardness of the oil-wax oelogel was much enhanced when a high Tm rice bran wax (RBW) or a high Tm sunflower wax (SFW) was combined with a low Tm berry wax (BEW) in oil. The synergistic property of the wax blend was discussed through crystal morphology and sequential crystallization of binary waxes, at which high Tm wax crystallized first with large size and low Tm wax crystallized later with much smaller size. The hardness enhancement of oil-binary wax blend system was explained through sintering effect at which small crystals formed bonding between large crystals of the high Tm waxes, creating a stronger cohesive oil-gel network.
Maru and Lahoti used sunflower wax (Tm=79°C) to replace the carnauba wax which was a hard wax in the standard lipstick formulation [4]. Their results showed that the quality of the lipstick containing sunflower wax was the same as the one containing the carnauba wax. The hardness of new lipstick was reduced and the deposit increased but not significantly.
Budai et al., studied the hardness of the lipsticks containing natural oils and natural waxes [6]. The natural oils were sunflower oil, castor oil, jojoba oil, and coconut oil. These oils were blended with either carnauba wax, or candelilla wax or yellow beeswax at various ratios. The results showed that the coconut oil, jojoba oil and carnauba wax had the strongest influence on the thermal parameters of the sticks such as softening point and drop point. The compression strength or hardness of the sticks could be achieved even with only 1% to 2% for both candelilla and carnauba waxes.
2.3.2 Factors Affecting Oil-Wax Structures: Oil Polarity
Another way to improve the hardness of oil-wax gel structure is to utilize the crystallization of wax in polar oils. Oil type and oil polarity have strong influence on the solubility of waxes which will affect the crystallization kinetics, crystal morphology, crystal size, crystal density and hardness of the oil-wax solid systems. For example, Imai and coworkers investigated the effect of various oil polarities on the hardness of the paraffin wax in oil-wax systems. They found that by increasing the polarity of oil, the gels became harder because the surfaces of crystals were rougher and the interconnection between wax crystals was tighter in the harder oil-wax gels, as revealed
