International Journal of Cosmetic Science, 2015, 37, 417–424
doi: 10.1111/ics.12213
The influence of propolis on rheological properties of lipstick U. Goik*, A. Ptaszek* and T. Goik† *Faculty of Food Technology, University of Agriculture in Krakow, ul. Balicka 122, 30-149 Krakow, Poland and †Faculty of Mechanical Engineering, Cracow University of Technology, al. Jana Pawła II 37, 31-864 Krakow, Poland
Received 2 October 2014, Accepted 7 February 2015
Keywords: argan oil, beeswax, lipsticks, propolis, rheology
Synopsis OBJECTIVE: The aim of this work was to study the effect of propolis on the rheological and textural properties of lipsticks. The studied lipsticks were based on raw materials and contained no synthetic compounds, preservatives, fragrances or dyes. The rheological and textural properties of the prepared lipsticks, both with and without propolis, were studied as a function of temperature and storage period. METHOD: Measurements were taken using an RS6000 rheometer (Haake, Germany) with a cone–plate sensor. The cone parameters were as follows: diameter 35 mm and angle 2°. Textural tests were performed using the same cone–plate geometry. RESULTS: The research results of rheological and textural properties of lipsticks, with and without the addition of propolis, indicate the possibility of application of propolis as a beneficial additive to such type of cosmetics. The presence of propolis does not significantly alter the viscoelastic properties of the lipsticks. The courses of flow curves indicate shear thinning, which is very advantageous from an application point of view. From the rheological point of view, the properties of lipsticks tested in low deformation conditions show some structural changes, most likely due to consolidation of the structure. CONCLUSION: The analysis of textural properties indicates that lipsticks with added propolis are more brittle and prone to crushing. However, the temperature increase (30°C) does not cause significant changes to the textural characteristics of these lipsticks. sume Re OBJECTIF: L’objectif de ce travail etait d’etudier l’effet de la propo levres. lis sur les proprietes rheologiques et de texture de rouges a Les rouges a levres etudies ne contenaient pas de composes synthetiques, des conservateurs, des parfums ou des colorants. Les pro levres prepares, avec prietes rheologiques et texturales des rouges a ou sans propolis, ont ete etudiees en fonction de la temperature et de la periode de stockage. l’aide d’un rheometre METHODE: Des mesures ont ete prises a RS6000 (Haake, Allemagne) avec un capteur c^ one-plan. Les parametres de c^ one ont ete les suivants: diametre 35 mm, angle de 2°. Les essais de texture ont ete realises avec la m^eme geometrie c^one-plan. RESULTATS: Les resultats de la recherche de proprietes rheologi levres, avec ou sans l’addition de ques et de texture des rouges a Correspondence: Urszula Goik, Faculty of Food Technology, University of Agriculture in Krakow, ul. Balicka 122, 30-149 Krakow, Poland. Tel.: (+48) 12 662 47 63; fax: (+48) 12 662 47 61; e-mail: u.goik@ ur.krakow.pl
propolis, indiquent la possibilite d’application du propolis en tant qu’additif avantageux pour ce type de produits cosmetiques. La presence de propolis ne modifie pas de facon significative les pro levres. Les cours de courbes prietes viscoelastiques des rouges a d’ecoulement indiquent une fluidification par cisaillement, qui est tres avantageux d’un point de vue de l’application. Du point de vue levres testes dans des condirheologique, les proprietes de rouges a tions de deformation faibles montrent certains changements structurels, probablement en raison de la consolidation de la structure. CONCLUSION: L’analyse des proprietes de texture indique que levres avec le propolis ajoute sont plus fragiles et sujles rouges a l’ecrasement. Cependant, l’augmentation de la temperature ettes a (30°C) ne provoque pas d’importantes modifications aux caracteris levres. tiques de texture de ces rouges a Introduction The term ‘lipstick’ is understood as a dispersion of dyes and pigments in an oil–fat–wax base, which is used in the make-up of the lips [1]. The lipstick gives the lips a certain colour, improves their look and protects them from external conditions. Technological developments allow the production of new types of lipsticks, for example protective, moisturizing, water and touch proof, herbal, totally transparent [2], and with added emulsion. Usually, the lipsticks are produced in the form of a stick, which should be smooth, glossy, flawless and temperature-resistant, and should not crack, shatter, break, smear or deform. Dyes and pigments should be equally dispersed throughout. Additionally, the lipstick should not irritate or cause allergic reactions, but should provide care and protection for the lips, form an oily film and be easy in application and removal [1–4]. The base components of lipstick recipes are oils and waxes. The most important parameter affecting the lipstick’s usage is its consistency, which is decisive with regard to application. Therefore, waxes characterized by high melting temperature are usually used in lipsticks as a base, for example carnauba, candelilla wax and beeswax. Waxes are responsible for hardness and viscosity, and they also increase the lipsticks’ melting temperature. Beeswax is a product containing over 300 different substances [5]. It is composed of higher fatty acid esters and alcohols, and it also contains small amounts of hydrocarbons, acids and other substances, as well as over 50 fragrance compounds [6]. It is widely used in the pharmaceutical, cosmetic and perfume industries. In cosmetic products, beeswax is mostly used as an emulsifier, providing the required plasticity and anti-bacterial and anti-fungal properties. Because of their occlusive, greasing and protective
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properties, plant oils are other common ingredients of lipstick recipes. Plant oils, such as argan oil (obtained from argan nut trees), are a rich source of fatty acids, especially polyunsaturated fatty acids (PUFAs). Argan oil has many valuable properties, due to its great variety of components: fatty acids (palmitic acid, stearic acid) including essential fatty acids (linoleic acid, c-linolenic acid, a-linolenic acid, arachidonic acid), tocopherols (a-tocopherol, dtocopherol, c-tocopherol), phenolic compounds (ferulic acid, vanillic acid, syringic acid, gallic acid, epicatechin, protocatechuic acid, isorhoifolin, catechin, hyperoside, isoquercitrin, hesperidin), sterols (schottenol, spinasterol) [7, 8], saponins [9] and squalene. It is used in cosmetic products to remedy dry, sensitive, easily irritable and allergic skin. Due to the presence of tocopherols and polyphenols, which act as antioxidants, argan oil stimulates intracellular respiration processes, neutralizes free radicals and protects the skin from the harmful effects of solar radiation. Polyunsaturated fatty acids are essential elements of the human diet and have an influence on human health. They are present in the skin and play an important role in maintaining the skin’s good condition. They are also components of the cellular membrane lipids and take part in the synthesis of eicosanoids, which play a regulatory role during inflammation. Additionally, various care and protective substances are added to lipsticks (e.g. panthenol, vitamin E), as well as antioxidants, preservatives, fragrances, dyes and pigments. Propolis is a resinous and sticky substance, produced by bees from sap and other botanical sources and used to seal the beehive. Propolis consistency is hard at the temperature of 15°C; at 36°C, it is soft and malleable; and at temperatures of 70–80°C, it becomes liquid [10]. Propolis and its extracts are characterized by anti-bacterial, antifungal and anti-viral properties. The antibiotic capabilities, against both bacteria and fungi, are exhibited by two groups of compounds: flavonoids and phenolic esters. The most potent antimicrobial substances are pinocembrin, galangin, pinostrobin, apigenin, myricetin and caffeic acid ethyl ester. Gallic acid is also characterized by high antibiotic activity [11–14]. In cosmetics, propolis is used as a preservative, an anti-bacterial and anti-dandruff agent, and is characterized by low toxicity and good compatibility with the skin, despite the risk of allergic reaction [15]. Nowadays, we increasingly avoid the use of synthetic compounds in the cosmetic and food industries. Synthetic compounds are usually replaced by human- and environment-friendly compounds [3, 4, 16, 17]. Due to frequent consumption by customers, the lipsticks are also tested for harmful substances such as lead, chromium and cadmium [18– 23]. The aim of this work was to prepare protective lipsticks, based on raw materials, and containing no synthetic compounds, preservatives, fragrances and dyes. There are no many publications concerning studies of the rheological and textural properties of these types of lipsticks. Most literature sources involve the determining of yield stress, the studies of thixotropic properties and the influence of temperature on the apparent viscosity of lipsticks [24]. Furthermore, there are studies on the development of the composition of lipsticks in terms of their functionality [25], and research on the impact of production conditions on lipsticks’ physical properties, mainly using image analysis [26–28]. Sensory analysis is also used in studies on textural properties of lipsticks [29]. However, in the literature, there seems to be a lack of reports describing full rheological characteristics of lipsticks. For these reasons, we decided to study the rheological and textural properties of the prepared lipsticks, particularly as functions of temperature and the duration of storage.
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Materials and methods The recipe of lipsticks The recipe for a protective lipstick based only on raw oil–wax components was prepared and used to produce two identical sets of lipsticks. One of these sets was additionally prepared with propolis addition, 10 g kg1. These lipsticks contained the following ingredients: argan oil (Argania spinosa oil) from roasted argan grains (BioPlanete, France) 304.29 g kg1, beeswax from bee farm 97.14 g kg1 and carnauba wax (Brazil) 40 g kg1 – one of the hardest plant waxes. Carnauba wax is used in small amounts of ~3– 5% in the production of lipsticks, correctors and emulsions. It contains mainly esters of cerotic and carnauba acids, and paraffin hydrocarbons. Its melting temperature is around 83°C. It mixes easily with a great variety of waxes; hence, its use as a base in production of lipsticks and pencils, where it improves the glossiness, increases hardness and raises their melting temperature, decreases the evaporation of volatile substances and increases the binding of oil substances in the product. Cocoa butter (Theobroma Cacao Seed Butter of Pharma Cosmetics) 278.57 g kg1 is another ingredient containing saturated fatty acids (palmitic acid, stearic acid) and, from the unsaturated acids group, oleic acid. Cocoa butter is hard and resistant to auto-oxidation; it heals inflammatory skin conditions and sunburn. It has good conditioning properties, and it moisturizes and improves skin tone by influencing microcirculation; moreover, its ingredients take part in the healing processes in the epidermis, reinforce cell membrane structures and protect them from water loss. Lanolin’s (280 g kg1, from Coel company, Krakow, Poland) chemical composition and physico-chemical properties closely resemble those of the lipids in human skin and hair. Any lanolin contains sterol esters, lanolin alcohols, lanolin acids and lanolin hydrocarbons [30]. Moreover, lanolin contains a significant unsaponifiable fraction (cholesterol, other sterols, cetyl alcohol and other lipid alcohols). Lanolin is used as a supporting component; when used in different cosmetic products, it regulates the skin’s water balance and can improve transition through the skin, due to compatibility with certain components. In cosmetic products, lanolin is often used in conjunction with cocoa butter, as together they work in synergy: cocoa butter neutralizes lanoline’s viscosity, whereas lanolin enables cocoa butter’s transition through the skin [31, 32]. All components were measured on the analytical laboratory scale and heated in a water bath to the temperature of 75–80°C; they were then stirred with a magnetic stirrer (250 rotations min1) for ~30 min. At the end of the stirring process, the propolis was added to one set of the oil–wax components mixture. Afterwards, the mixture was poured into moulds, formed into 7g lipsticks (13 mm in diameter) and, finally, cooled. The lipsticks, with and without propolis, were then subjected to rheological tests at temperatures of 27, 32 (skin application temperature), 37 and 42°C. The remaining lipsticks were stored at room temperature (20°C) and again rheological tests were performed after 1, 2, 3 and 4 weeks at the same temperatures. The selection of 20°C as a temperature level and a 4-week storage period was based on the raw material composition of the tested samples and actual conditions of use, handling and application of this type of lipsticks by consumers. The tested lipsticks were prepared, without the use of any preservatives, from fatty materials, which are subject to rancidity. Formulas of this type have short shelf life and it is advisable to store them in the refrigerator. Lipsticks, without any preservatives, undergoing a regular degradation process will exhibit structural changes in time,
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and therefore, their rheological properties will also change. A selection of lipsticks was also stored in a vacuum dryer for 10 days at 40°C, to speed up the ageing process of the product. After being taken out of the dryer, the lipsticks, both with and without propolis, did not reveal any changes to their external appearance.
(s0) was defined as a point, where the two defined lines intersect in the (log (s0) – log (c0)) coordinate system. The point characterizes the curvature of the relationship. For all analysed cases, the range of applied stress was determined, and the whole analysis was carried out for 300 s.
Rheology
Flow curves
All measurements were taken with an Haake RS6000 rheometer (Thermo Fisher Scientific Inc., Karlsruhe, Germany), using a cone– plate sensor. The cone parameters were as follows: diameter 35 mm and angle 2°. The measurement gap size was determined based on preliminary studies and adjusted to 0.5 mm. This was the highest value at which repeatability of 95% was achieved.
The flow curves were measured within the shear rates ranging from 101 s1 to 103 s1. The research was performed both in the increasing (flow curve ‘up’) and in the decreasing (flow curve ‘down’) shear rate mode. In both cases, the experiment lasted 300 s. The results obtained this way can also be interpreted as a quality test of the hysteresis loop. A Cross state equation was fitted to the obtained data.
Linear viscoelasticity Rheological studies relied on the measurement of the values of the complex modulus G* as a function of frequency, within the range of 0.1–10 Hz at 20°C. The first step was to determine the area of linear viscoelasticity [33]. This test relies, in case of frequency domain measurements, on measuring the absolute value of the complex modulus as the function |G*(co)| of deformation amplitude at constant frequency. The linear viscoelasticity area determines the range of deformation amplitudes (co) for which G*(co) values are parallel to the axis of the abscissa (co axis). Analysed for all the systems, the measurements of linear viscoelasticity range were taken at extreme frequency values. It allowed us to determine the value of co = 0.01, common for all the systems, which was used in the subsequent sample tests. For each tested lipstick, we measured the G0 (x) and G″( x) values in triplicate, at 20°C. Yield stress The method for the determining yield stress consisted of subjecting the studied material to stress, increased linearly in time and observing the deformations. Subsequently, the value of the yield stress
Textural properties Textural tests were performed using the same cone–plate geometry, with the gap decreasing linearly from 15 to 5 mm, in the time range of 40 s. The changes of normal forces (FN, Pa) were measured. Discussion Rheological properties Lipstick tests revealed viscoelasticity properties which are characteristic for the flow area (Fig. 1). At lower temperatures (37°C), the values of loss modulus G″ are higher than those of storage modulus G0 . It is possible to scale the moduli values against temperature. As a result, a master curve was obtained and the values for the scaling coefficients were as follows: 1.0 for data obtained at the reference temperature of 27°C, 0.5 for 32°C and 0.05 for 37°C. The ability to scale the viscoelastic properties indicates their stability within those temperatures and excludes the possibility of structural changes to the lipsticks. Increasing the temperature to 42°C
Figure 1 G0 and G″ as a function of frequency (x) for lipsticks containing propolis, after preparation. Experimental data at three temperatures were scaled using time–temperature superposition (values of scaling coefficients aT shown in the figure).
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changes their rheological characteristics. Modulus values are significantly lower and practically equal within 1–50 Hz frequency. The G0 (x) and G″(x) correlation for 42°C equals the moduli intersection point, where the flow area ends, but it is not present for just one specific frequency; rather, it can be observed within a range. The moduli graphs differ from the discussed master curve, which suggests qualitative changes caused by the temperature increase. The obtained results indicate that the tested lipsticks are structurally stable over a wide range of temperatures. They exhibit the greatest tendency to flow at the highest test temperature of 42°C. Storage of the lipsticks at a temperature of 20°C changes the outline of the viscoelastic properties (Fig. 2). The values of G0 (x) and G″(x) are not subject to temperature scaling. The G0 values obtained for 27 and 32°C are similar; they rise in function of frequency, reaching a maximum, after which the G0 and G″ values start to decrease. The corresponding G″ values mark a different course, so that at 27°C initially G0 >G″, and then, we observe an intersection of G0 (x) and G″ (x). In the case of data obtained at 32°C, the values of G0 and G″ are similar, and above x = 20 Hz, G″ is higher than G0 . Further temperature increase leads to an evident decrease in the moduli values. The graph representation of conservative and loss moduli at 42°C might be characteristic of the plateau area. After a 4-week storage period, the temperature scaling of the viscoelastic properties of lipsticks is impossible. The tests of viscoelastic properties in the linear range are carried out under low strain conditions and therefore allow the observation of the lipsticks’ structural characteristics. The obtained results indicate that certain changes occur in the lipsticks’ structure during storage, which is manifested by a reduction of the values for the storage and loss moduli. At the same time, a shift in the character of the viscoelastic properties (G0 >G″) was observed. These observations only concern the lipsticks with the addition of propolis (Figs 1 and 2); they were not observed in the case of lipsticks without propolis. The next figure (Fig. 3) illustrates the influence of temperature and time on the values of yield stress for the tested lipsticks. In the case of lipsticks tested at 27°C and 32°C (immediately after prepa-
ration), we can observe the occurrence of double yield stress. There is a visible increase of deformation after crossing the stress level of 55 Pa and again at 600 Pa for 27°C. After 4 weeks’ storage, we can observe a difference in the value of s0. At 27°C, the yield stress increases up to 70 and 670 Pa. At the temperature of 32°C, we can observe a different situation: the first yield stress value goes down to 20 Pa and the second value remains practically constant. At the temperatures of 37 and 42°C, within the tested range of deformations, we observe one yield stress value for each: 200 and 50 Pa, respectively. The storage period has no influence on the s0 values for these temperatures. The rheological properties of lipsticks in the flow area are shown in the graphs illustrating the flow curves subjected to temperature scaling [33]. As a reference, we adopted the apparent viscosity values obtained at the 27°C (aT = 1). Scaling of the apparent viscosity values was possible for both sets of lipsticks: with and without the addition of propolis. All tested lipsticks, both with (Fig. 4 top) and without (Fig. 4 bottom) propolis, exhibit the presence of shear thinning. Lipsticks’ apparent viscosity decreases in the range of 100–0.1 Pas. With temperature increase, we can observe the shear-thinning phenomenon. For lipsticks without propolis, the apparent viscosity abruptly decreases at 27 and 32°C. Then at 37 and 42°C, the difference is not as visible. After 4 weeks of storage, the situation is reversed. Temperature changes from 27 to 32°C cause very little apparent viscosity changes. Further increase of temperature from 37 to 42°C leads to more evident viscosity changes. Furthermore, for the analysed systems, a temperature raise causes a decrease of the hysteresis effect (Table I) for lipsticks just after preparation, as well as those after 4 weeks storage. For the stored lipsticks, the hysteresis phenomenon was less intensive, and the change of energy values between extreme temperature levels was not as visible, as for the systems just after preparation. The addition of propolis causes viscosity’s dependence on temperature (in the studied range of shear rate), to decrease less dramatically, than in the cases without propolis. Only at the temperature of 42°C, a lower apparent viscosity value was observed in comparison with the previous case. After the storage
Figure 2 G0 and G″ as a function of frequency (x) for lipsticks containing propolis, after 4 weeks of storage.
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Figure 3 Yield stress determination method. Presented data were obtained for lipsticks containing propolis just after preparation and after 4 weeks of storage at room temperature.
Figure 4 Viscosity flow curves at different temperatures for lipsticks without propolis (first row) and with propolis (second row). In the two columns are presented properties of lipsticks after preparation (0-) and after 4 weeks of storage (4-).
period, the temperature-dependent flow curves are clearly separated into two groups. In the first group, for 27 and 32°C, the viscosity values within the range of shear rate are close. In the second group, for temperatures of 37 and 42°C, for small shear rate values, viscosity is almost identical for both temperatures. At 30 s1, an evident difference can be observed between the apparent viscosity values, and at higher temperatures, the viscosity decreases more rapidly as a function of shear rate. The hysteresis phenomenon (represented by the amount of dissipated energy) for lipsticks without propolis decreases, as the temperature raises (Table I). For the lipsticks with propolis, at the temperature of 32°C, we observe a maximum of the value of energy dispersed by the systems. The Cross model was fitted for all the measured data (Table I), and the values of time constant s and the m flow index were
estimated. The flow index values do not depend in any way on the addition of propolis or the lipsticks’ storage period (Table I). We can observe the changes in the value of the time constant s. The temperature raise causes the increase of the s value. This is a result of the fact that lipsticks’ viscosity decreases and they become more fluid. The value increase of the time constant reflects this phenomenon. For the lipsticks with and without propolis, tested on the day of preparation, s values are close to one another and fluctuate in the range of 6.2–25.1 s. The storage of the lipsticks for 4 weeks causes slight changes to the flow curves (Fig. 4); however, the evidently lower s values are a result of lipsticks more elastic character. The results of oscillatory tests confirm the above observations (Fig. 2). Lipsticks tested just after being formulated reveal properties characteristic for flow area (G″>G0 ), and those tested
© 2015 Society of Cosmetic Scientists and the Societe Francaise de Cosmetologie International Journal of Cosmetic Science, 37, 417–424
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Influence of propolis on the rheology of lipstick Table I Values of Cross model parameters and amount of dissipated energy DE (samples volume 0.4 9 106 m3) Week of storage
Lipstick without propolis 0
4
Lipstick with propolis 0
4
T, °C
s, s
m
DE 3 103 mJ
27 32 37 42 27 32 37 42
6.2 9.5 15.5 25.1 1.25 1.5 3.15 4.5
1.2
84.4 61.7 22.5 13.8 61.2 65.1 28.5 25.3
27 32 37 42 27 32 37 42
7.6 15 25 25 2 1.2 2.5 4.1
1.2
highlights the effect of the addition of propolis. The values of aT coefficient for lipsticks with propolis (Fig. 3) are considerably higher, which indicates a greater differentiation of apparent viscosity as a function of temperature. Textural properties
1.5
1.5
41.9 39.8 14.8 9.9 44.6 64.9 34.5 17.5
after 4 weeks exhibit different properties. G0 and G″ values become equal for temperatures of 32 and 37°C and G0 >G″ for temperatures of 27 and 42°C. Temperature scaling of apparent viscosities of the lipsticks reveals that both propolis addition and storage period affect the rheological properties within the flow area. For lipsticks with propolis, the apparent viscosity requires scaling with the use of aT factors with lower values than aT factors obtained for lipsticks without the addition of propolis. This means that the effect of temperature on their apparent viscosity is weaker, in comparison with the lipsticks without propolis. For both types of lipsticks, the 4-week storage period changes the values of the scaling factors aT and
The next figure (Fig. 5) illustrates the results of the textural tests of lipsticks with and without (control) propolis, performed at 20°C on the day of preparation. Additionally, we tested the properties of a lipstick with propolis, stored for approximately 5 h at a temperature of 32°C. The lipstick without propolis is characterized by the lowest fragility (4.2 Pa) and the maximum hardness (5 Pa). At the same time, it is the most elastic, as crushing occurs only after reducing the distance of the sensor to h = 11.8 mm. It means that under these conditions, the lipstick is deformed without causing cracks. For the lipstick with propolis, the crushing occurs at h = 12.8 mm; this requires the use of forces in the range of 2.5–3 Pa, which is slightly lower than in the control lipstick. This may indicate that the lipsticks with propolis have limited elastic properties and can crack more easily. Further operation of the sensor leads to the crushing of the lipsticks, and the obtained crushing force values vary for the tested lipsticks and are as follows: for the control lipstick the crushing force value is 5 Pa, 4.5 Pa for lipsticks at the temperature of 20°C and 3.8 Pa for the lipstick heated to the temperature of 32°C. The differences in the properties of lipsticks with propolis, stored at 20 and 32°C, are insignificant. Conclusions The rheological characteristics of lipsticks provide valuable information about the product’s usage parameters, for example contact time with the skin, where the lipstick was applied. They are described by a number of parameters, amongst which yield stress and apparent viscosity are the most important. A high yield stress
Figure 5 Graphical representation of textural tests of lipsticks. Solid line represents decreasing sensor distance as a function of time.
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value may cause difficulty in the distribution of the product onto the skin. For the tested lipsticks, the obtained values of s0 are highest at 27°C, and they decrease as the temperature rises. Four-week storage period of the lipsticks affects the s0 values at lower temperatures. From the application point of view, neither temperature increase nor 4-week storage time causes degradation of the lipsticks’ properties. This allows us to assume that temperature changes occurring during lipsticks’ storage in consumer conditions will not cause mechanical damage to the product. Another important rheological parameter, from the application point of view, is the apparent viscosity. A high value of the apparent viscosity ensures product’s longer contact time with its place of application, which is beneficial in the case of lipsticks. The extended contact time with the skin is an advantage, because the lipstick forms an occlusive protective layer on the skin. The temperature conditions on the skin’s surface (higher temperature) require the creation of a product, which would be characterized not only by high apparent viscosity, but also by appropriate thixotropic properties. Propolis
addition to the tested lipsticks reduces the hysteresis effect of the flow curves (particularly at higher temperatures). The properties characteristic of shear-thinning systems ensures the appropriate application of the lipstick onto the skin. The reduction of the thixotropy effect, by addition of propolis to the lipstick, allows for better reconstruction of the cosmetic’s structure after the shear forces are removed – that is after the lipstick has been applied to the skin. For this reason, the addition of propolis has beneficial effects on application properties of the studied lipsticks. The analysis of textural properties of the lipsticks indicates that lipsticks with propolis are more brittle and prone to crushing. However, the temperature increase (30°C) does not cause significant changes in the textural characteristics of these lipsticks. Acknowledgement Research funded by grants for statutory activities for the year 2014 granted by Polish Ministry of Science and Higher Education.
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New triterpenoid saponins from Argania spinosa. J. Nat. Med. 61, 375–380 (2007). Encyklopedia pszczelarska. PWRiL, Warszawa (1989). Castaldo, S. and Capasso, F. Propolis, an old remedy used in modern medicine. Fitoterapia 73, 1–6 (2002). Kez dzia, B. and Hołderna-Kez dzia, E. Aktywnosc antybiotyczna propolisu krajowego i europejskiego. Postezpy Fitoterapii. 2, 97– 107 (2013). Kez dzia, B. Skład chemiczny i aktywnosc biologiczna propolisu pochodzaz cego z r oz_ nych rejon ow swiata. Postez py Fitoterapii. 1, 23– 35 (2006). Xu, Y., Luo, L., Chen, B. and Fu, Y. Recent development of chemical components in propolis. Front. Biol. China 4, 385–391 (2009). Bogdanov, S. Propolis: Composition, health, medicine: A Review (2012). Available at: www.bee-hexagon.net Kamairudin, N., Abd Gani, S.S., Fard Masoumi, H.R. and Hashim, P. Optimization of natural lipstick formulation based on pitaya (Hylocereus polyrhizus) seed oil using D-optimal mixture experimental design. Molecules 19, 16672–16683 (2014). Rajin, M., Bono, A. and Mun, H.Ch. Optimisation of natural ingredient based lipstick formulation by using mixture design. J. Appl. Sci. 7, 2099–2103 (2007). Al-Saleh, I., Al-Enazi, S. and Shinwari, N. Assessment of lead in cosmetic products. Regul. Toxicol. Pharmacol. 54, 105–113 (2009). Piccinini, P., Piecha, M. and Torrent, S.F. European survey on the content of lead in
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doi: 10.1111/ics.12213
The influence of propolis on rheological properties of lipstick U. Goik*, A. Ptaszek* and T. Goik† *Faculty of Food Technology, University of Agriculture in Krakow, ul. Balicka 122, 30-149 Krakow, Poland and †Faculty of Mechanical Engineering, Cracow University of Technology, al. Jana Pawła II 37, 31-864 Krakow, Poland
Received 2 October 2014, Accepted 7 February 2015
Keywords: argan oil, beeswax, lipsticks, propolis, rheology
Synopsis OBJECTIVE: The aim of this work was to study the effect of propolis on the rheological and textural properties of lipsticks. The studied lipsticks were based on raw materials and contained no synthetic compounds, preservatives, fragrances or dyes. The rheological and textural properties of the prepared lipsticks, both with and without propolis, were studied as a function of temperature and storage period. METHOD: Measurements were taken using an RS6000 rheometer (Haake, Germany) with a cone–plate sensor. The cone parameters were as follows: diameter 35 mm and angle 2°. Textural tests were performed using the same cone–plate geometry. RESULTS: The research results of rheological and textural properties of lipsticks, with and without the addition of propolis, indicate the possibility of application of propolis as a beneficial additive to such type of cosmetics. The presence of propolis does not significantly alter the viscoelastic properties of the lipsticks. The courses of flow curves indicate shear thinning, which is very advantageous from an application point of view. From the rheological point of view, the properties of lipsticks tested in low deformation conditions show some structural changes, most likely due to consolidation of the structure. CONCLUSION: The analysis of textural properties indicates that lipsticks with added propolis are more brittle and prone to crushing. However, the temperature increase (30°C) does not cause significant changes to the textural characteristics of these lipsticks. sume Re OBJECTIF: L’objectif de ce travail etait d’etudier l’effet de la propo levres. lis sur les proprietes rheologiques et de texture de rouges a Les rouges a levres etudies ne contenaient pas de composes synthetiques, des conservateurs, des parfums ou des colorants. Les pro levres prepares, avec prietes rheologiques et texturales des rouges a ou sans propolis, ont ete etudiees en fonction de la temperature et de la periode de stockage. l’aide d’un rheometre METHODE: Des mesures ont ete prises a RS6000 (Haake, Allemagne) avec un capteur c^ one-plan. Les parametres de c^ one ont ete les suivants: diametre 35 mm, angle de 2°. Les essais de texture ont ete realises avec la m^eme geometrie c^one-plan. RESULTATS: Les resultats de la recherche de proprietes rheologi levres, avec ou sans l’addition de ques et de texture des rouges a Correspondence: Urszula Goik, Faculty of Food Technology, University of Agriculture in Krakow, ul. Balicka 122, 30-149 Krakow, Poland. Tel.: (+48) 12 662 47 63; fax: (+48) 12 662 47 61; e-mail: u.goik@ ur.krakow.pl
propolis, indiquent la possibilite d’application du propolis en tant qu’additif avantageux pour ce type de produits cosmetiques. La presence de propolis ne modifie pas de facon significative les pro levres. Les cours de courbes prietes viscoelastiques des rouges a d’ecoulement indiquent une fluidification par cisaillement, qui est tres avantageux d’un point de vue de l’application. Du point de vue levres testes dans des condirheologique, les proprietes de rouges a tions de deformation faibles montrent certains changements structurels, probablement en raison de la consolidation de la structure. CONCLUSION: L’analyse des proprietes de texture indique que levres avec le propolis ajoute sont plus fragiles et sujles rouges a l’ecrasement. Cependant, l’augmentation de la temperature ettes a (30°C) ne provoque pas d’importantes modifications aux caracteris levres. tiques de texture de ces rouges a Introduction The term ‘lipstick’ is understood as a dispersion of dyes and pigments in an oil–fat–wax base, which is used in the make-up of the lips [1]. The lipstick gives the lips a certain colour, improves their look and protects them from external conditions. Technological developments allow the production of new types of lipsticks, for example protective, moisturizing, water and touch proof, herbal, totally transparent [2], and with added emulsion. Usually, the lipsticks are produced in the form of a stick, which should be smooth, glossy, flawless and temperature-resistant, and should not crack, shatter, break, smear or deform. Dyes and pigments should be equally dispersed throughout. Additionally, the lipstick should not irritate or cause allergic reactions, but should provide care and protection for the lips, form an oily film and be easy in application and removal [1–4]. The base components of lipstick recipes are oils and waxes. The most important parameter affecting the lipstick’s usage is its consistency, which is decisive with regard to application. Therefore, waxes characterized by high melting temperature are usually used in lipsticks as a base, for example carnauba, candelilla wax and beeswax. Waxes are responsible for hardness and viscosity, and they also increase the lipsticks’ melting temperature. Beeswax is a product containing over 300 different substances [5]. It is composed of higher fatty acid esters and alcohols, and it also contains small amounts of hydrocarbons, acids and other substances, as well as over 50 fragrance compounds [6]. It is widely used in the pharmaceutical, cosmetic and perfume industries. In cosmetic products, beeswax is mostly used as an emulsifier, providing the required plasticity and anti-bacterial and anti-fungal properties. Because of their occlusive, greasing and protective
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properties, plant oils are other common ingredients of lipstick recipes. Plant oils, such as argan oil (obtained from argan nut trees), are a rich source of fatty acids, especially polyunsaturated fatty acids (PUFAs). Argan oil has many valuable properties, due to its great variety of components: fatty acids (palmitic acid, stearic acid) including essential fatty acids (linoleic acid, c-linolenic acid, a-linolenic acid, arachidonic acid), tocopherols (a-tocopherol, dtocopherol, c-tocopherol), phenolic compounds (ferulic acid, vanillic acid, syringic acid, gallic acid, epicatechin, protocatechuic acid, isorhoifolin, catechin, hyperoside, isoquercitrin, hesperidin), sterols (schottenol, spinasterol) [7, 8], saponins [9] and squalene. It is used in cosmetic products to remedy dry, sensitive, easily irritable and allergic skin. Due to the presence of tocopherols and polyphenols, which act as antioxidants, argan oil stimulates intracellular respiration processes, neutralizes free radicals and protects the skin from the harmful effects of solar radiation. Polyunsaturated fatty acids are essential elements of the human diet and have an influence on human health. They are present in the skin and play an important role in maintaining the skin’s good condition. They are also components of the cellular membrane lipids and take part in the synthesis of eicosanoids, which play a regulatory role during inflammation. Additionally, various care and protective substances are added to lipsticks (e.g. panthenol, vitamin E), as well as antioxidants, preservatives, fragrances, dyes and pigments. Propolis is a resinous and sticky substance, produced by bees from sap and other botanical sources and used to seal the beehive. Propolis consistency is hard at the temperature of 15°C; at 36°C, it is soft and malleable; and at temperatures of 70–80°C, it becomes liquid [10]. Propolis and its extracts are characterized by anti-bacterial, antifungal and anti-viral properties. The antibiotic capabilities, against both bacteria and fungi, are exhibited by two groups of compounds: flavonoids and phenolic esters. The most potent antimicrobial substances are pinocembrin, galangin, pinostrobin, apigenin, myricetin and caffeic acid ethyl ester. Gallic acid is also characterized by high antibiotic activity [11–14]. In cosmetics, propolis is used as a preservative, an anti-bacterial and anti-dandruff agent, and is characterized by low toxicity and good compatibility with the skin, despite the risk of allergic reaction [15]. Nowadays, we increasingly avoid the use of synthetic compounds in the cosmetic and food industries. Synthetic compounds are usually replaced by human- and environment-friendly compounds [3, 4, 16, 17]. Due to frequent consumption by customers, the lipsticks are also tested for harmful substances such as lead, chromium and cadmium [18– 23]. The aim of this work was to prepare protective lipsticks, based on raw materials, and containing no synthetic compounds, preservatives, fragrances and dyes. There are no many publications concerning studies of the rheological and textural properties of these types of lipsticks. Most literature sources involve the determining of yield stress, the studies of thixotropic properties and the influence of temperature on the apparent viscosity of lipsticks [24]. Furthermore, there are studies on the development of the composition of lipsticks in terms of their functionality [25], and research on the impact of production conditions on lipsticks’ physical properties, mainly using image analysis [26–28]. Sensory analysis is also used in studies on textural properties of lipsticks [29]. However, in the literature, there seems to be a lack of reports describing full rheological characteristics of lipsticks. For these reasons, we decided to study the rheological and textural properties of the prepared lipsticks, particularly as functions of temperature and the duration of storage.
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Materials and methods The recipe of lipsticks The recipe for a protective lipstick based only on raw oil–wax components was prepared and used to produce two identical sets of lipsticks. One of these sets was additionally prepared with propolis addition, 10 g kg1. These lipsticks contained the following ingredients: argan oil (Argania spinosa oil) from roasted argan grains (BioPlanete, France) 304.29 g kg1, beeswax from bee farm 97.14 g kg1 and carnauba wax (Brazil) 40 g kg1 – one of the hardest plant waxes. Carnauba wax is used in small amounts of ~3– 5% in the production of lipsticks, correctors and emulsions. It contains mainly esters of cerotic and carnauba acids, and paraffin hydrocarbons. Its melting temperature is around 83°C. It mixes easily with a great variety of waxes; hence, its use as a base in production of lipsticks and pencils, where it improves the glossiness, increases hardness and raises their melting temperature, decreases the evaporation of volatile substances and increases the binding of oil substances in the product. Cocoa butter (Theobroma Cacao Seed Butter of Pharma Cosmetics) 278.57 g kg1 is another ingredient containing saturated fatty acids (palmitic acid, stearic acid) and, from the unsaturated acids group, oleic acid. Cocoa butter is hard and resistant to auto-oxidation; it heals inflammatory skin conditions and sunburn. It has good conditioning properties, and it moisturizes and improves skin tone by influencing microcirculation; moreover, its ingredients take part in the healing processes in the epidermis, reinforce cell membrane structures and protect them from water loss. Lanolin’s (280 g kg1, from Coel company, Krakow, Poland) chemical composition and physico-chemical properties closely resemble those of the lipids in human skin and hair. Any lanolin contains sterol esters, lanolin alcohols, lanolin acids and lanolin hydrocarbons [30]. Moreover, lanolin contains a significant unsaponifiable fraction (cholesterol, other sterols, cetyl alcohol and other lipid alcohols). Lanolin is used as a supporting component; when used in different cosmetic products, it regulates the skin’s water balance and can improve transition through the skin, due to compatibility with certain components. In cosmetic products, lanolin is often used in conjunction with cocoa butter, as together they work in synergy: cocoa butter neutralizes lanoline’s viscosity, whereas lanolin enables cocoa butter’s transition through the skin [31, 32]. All components were measured on the analytical laboratory scale and heated in a water bath to the temperature of 75–80°C; they were then stirred with a magnetic stirrer (250 rotations min1) for ~30 min. At the end of the stirring process, the propolis was added to one set of the oil–wax components mixture. Afterwards, the mixture was poured into moulds, formed into 7g lipsticks (13 mm in diameter) and, finally, cooled. The lipsticks, with and without propolis, were then subjected to rheological tests at temperatures of 27, 32 (skin application temperature), 37 and 42°C. The remaining lipsticks were stored at room temperature (20°C) and again rheological tests were performed after 1, 2, 3 and 4 weeks at the same temperatures. The selection of 20°C as a temperature level and a 4-week storage period was based on the raw material composition of the tested samples and actual conditions of use, handling and application of this type of lipsticks by consumers. The tested lipsticks were prepared, without the use of any preservatives, from fatty materials, which are subject to rancidity. Formulas of this type have short shelf life and it is advisable to store them in the refrigerator. Lipsticks, without any preservatives, undergoing a regular degradation process will exhibit structural changes in time,
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and therefore, their rheological properties will also change. A selection of lipsticks was also stored in a vacuum dryer for 10 days at 40°C, to speed up the ageing process of the product. After being taken out of the dryer, the lipsticks, both with and without propolis, did not reveal any changes to their external appearance.
(s0) was defined as a point, where the two defined lines intersect in the (log (s0) – log (c0)) coordinate system. The point characterizes the curvature of the relationship. For all analysed cases, the range of applied stress was determined, and the whole analysis was carried out for 300 s.
Rheology
Flow curves
All measurements were taken with an Haake RS6000 rheometer (Thermo Fisher Scientific Inc., Karlsruhe, Germany), using a cone– plate sensor. The cone parameters were as follows: diameter 35 mm and angle 2°. The measurement gap size was determined based on preliminary studies and adjusted to 0.5 mm. This was the highest value at which repeatability of 95% was achieved.
The flow curves were measured within the shear rates ranging from 101 s1 to 103 s1. The research was performed both in the increasing (flow curve ‘up’) and in the decreasing (flow curve ‘down’) shear rate mode. In both cases, the experiment lasted 300 s. The results obtained this way can also be interpreted as a quality test of the hysteresis loop. A Cross state equation was fitted to the obtained data.
Linear viscoelasticity Rheological studies relied on the measurement of the values of the complex modulus G* as a function of frequency, within the range of 0.1–10 Hz at 20°C. The first step was to determine the area of linear viscoelasticity [33]. This test relies, in case of frequency domain measurements, on measuring the absolute value of the complex modulus as the function |G*(co)| of deformation amplitude at constant frequency. The linear viscoelasticity area determines the range of deformation amplitudes (co) for which G*(co) values are parallel to the axis of the abscissa (co axis). Analysed for all the systems, the measurements of linear viscoelasticity range were taken at extreme frequency values. It allowed us to determine the value of co = 0.01, common for all the systems, which was used in the subsequent sample tests. For each tested lipstick, we measured the G0 (x) and G″( x) values in triplicate, at 20°C. Yield stress The method for the determining yield stress consisted of subjecting the studied material to stress, increased linearly in time and observing the deformations. Subsequently, the value of the yield stress
Textural properties Textural tests were performed using the same cone–plate geometry, with the gap decreasing linearly from 15 to 5 mm, in the time range of 40 s. The changes of normal forces (FN, Pa) were measured. Discussion Rheological properties Lipstick tests revealed viscoelasticity properties which are characteristic for the flow area (Fig. 1). At lower temperatures (37°C), the values of loss modulus G″ are higher than those of storage modulus G0 . It is possible to scale the moduli values against temperature. As a result, a master curve was obtained and the values for the scaling coefficients were as follows: 1.0 for data obtained at the reference temperature of 27°C, 0.5 for 32°C and 0.05 for 37°C. The ability to scale the viscoelastic properties indicates their stability within those temperatures and excludes the possibility of structural changes to the lipsticks. Increasing the temperature to 42°C
Figure 1 G0 and G″ as a function of frequency (x) for lipsticks containing propolis, after preparation. Experimental data at three temperatures were scaled using time–temperature superposition (values of scaling coefficients aT shown in the figure).
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changes their rheological characteristics. Modulus values are significantly lower and practically equal within 1–50 Hz frequency. The G0 (x) and G″(x) correlation for 42°C equals the moduli intersection point, where the flow area ends, but it is not present for just one specific frequency; rather, it can be observed within a range. The moduli graphs differ from the discussed master curve, which suggests qualitative changes caused by the temperature increase. The obtained results indicate that the tested lipsticks are structurally stable over a wide range of temperatures. They exhibit the greatest tendency to flow at the highest test temperature of 42°C. Storage of the lipsticks at a temperature of 20°C changes the outline of the viscoelastic properties (Fig. 2). The values of G0 (x) and G″(x) are not subject to temperature scaling. The G0 values obtained for 27 and 32°C are similar; they rise in function of frequency, reaching a maximum, after which the G0 and G″ values start to decrease. The corresponding G″ values mark a different course, so that at 27°C initially G0 >G″, and then, we observe an intersection of G0 (x) and G″ (x). In the case of data obtained at 32°C, the values of G0 and G″ are similar, and above x = 20 Hz, G″ is higher than G0 . Further temperature increase leads to an evident decrease in the moduli values. The graph representation of conservative and loss moduli at 42°C might be characteristic of the plateau area. After a 4-week storage period, the temperature scaling of the viscoelastic properties of lipsticks is impossible. The tests of viscoelastic properties in the linear range are carried out under low strain conditions and therefore allow the observation of the lipsticks’ structural characteristics. The obtained results indicate that certain changes occur in the lipsticks’ structure during storage, which is manifested by a reduction of the values for the storage and loss moduli. At the same time, a shift in the character of the viscoelastic properties (G0 >G″) was observed. These observations only concern the lipsticks with the addition of propolis (Figs 1 and 2); they were not observed in the case of lipsticks without propolis. The next figure (Fig. 3) illustrates the influence of temperature and time on the values of yield stress for the tested lipsticks. In the case of lipsticks tested at 27°C and 32°C (immediately after prepa-
ration), we can observe the occurrence of double yield stress. There is a visible increase of deformation after crossing the stress level of 55 Pa and again at 600 Pa for 27°C. After 4 weeks’ storage, we can observe a difference in the value of s0. At 27°C, the yield stress increases up to 70 and 670 Pa. At the temperature of 32°C, we can observe a different situation: the first yield stress value goes down to 20 Pa and the second value remains practically constant. At the temperatures of 37 and 42°C, within the tested range of deformations, we observe one yield stress value for each: 200 and 50 Pa, respectively. The storage period has no influence on the s0 values for these temperatures. The rheological properties of lipsticks in the flow area are shown in the graphs illustrating the flow curves subjected to temperature scaling [33]. As a reference, we adopted the apparent viscosity values obtained at the 27°C (aT = 1). Scaling of the apparent viscosity values was possible for both sets of lipsticks: with and without the addition of propolis. All tested lipsticks, both with (Fig. 4 top) and without (Fig. 4 bottom) propolis, exhibit the presence of shear thinning. Lipsticks’ apparent viscosity decreases in the range of 100–0.1 Pas. With temperature increase, we can observe the shear-thinning phenomenon. For lipsticks without propolis, the apparent viscosity abruptly decreases at 27 and 32°C. Then at 37 and 42°C, the difference is not as visible. After 4 weeks of storage, the situation is reversed. Temperature changes from 27 to 32°C cause very little apparent viscosity changes. Further increase of temperature from 37 to 42°C leads to more evident viscosity changes. Furthermore, for the analysed systems, a temperature raise causes a decrease of the hysteresis effect (Table I) for lipsticks just after preparation, as well as those after 4 weeks storage. For the stored lipsticks, the hysteresis phenomenon was less intensive, and the change of energy values between extreme temperature levels was not as visible, as for the systems just after preparation. The addition of propolis causes viscosity’s dependence on temperature (in the studied range of shear rate), to decrease less dramatically, than in the cases without propolis. Only at the temperature of 42°C, a lower apparent viscosity value was observed in comparison with the previous case. After the storage
Figure 2 G0 and G″ as a function of frequency (x) for lipsticks containing propolis, after 4 weeks of storage.
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Figure 3 Yield stress determination method. Presented data were obtained for lipsticks containing propolis just after preparation and after 4 weeks of storage at room temperature.
Figure 4 Viscosity flow curves at different temperatures for lipsticks without propolis (first row) and with propolis (second row). In the two columns are presented properties of lipsticks after preparation (0-) and after 4 weeks of storage (4-).
period, the temperature-dependent flow curves are clearly separated into two groups. In the first group, for 27 and 32°C, the viscosity values within the range of shear rate are close. In the second group, for temperatures of 37 and 42°C, for small shear rate values, viscosity is almost identical for both temperatures. At 30 s1, an evident difference can be observed between the apparent viscosity values, and at higher temperatures, the viscosity decreases more rapidly as a function of shear rate. The hysteresis phenomenon (represented by the amount of dissipated energy) for lipsticks without propolis decreases, as the temperature raises (Table I). For the lipsticks with propolis, at the temperature of 32°C, we observe a maximum of the value of energy dispersed by the systems. The Cross model was fitted for all the measured data (Table I), and the values of time constant s and the m flow index were
estimated. The flow index values do not depend in any way on the addition of propolis or the lipsticks’ storage period (Table I). We can observe the changes in the value of the time constant s. The temperature raise causes the increase of the s value. This is a result of the fact that lipsticks’ viscosity decreases and they become more fluid. The value increase of the time constant reflects this phenomenon. For the lipsticks with and without propolis, tested on the day of preparation, s values are close to one another and fluctuate in the range of 6.2–25.1 s. The storage of the lipsticks for 4 weeks causes slight changes to the flow curves (Fig. 4); however, the evidently lower s values are a result of lipsticks more elastic character. The results of oscillatory tests confirm the above observations (Fig. 2). Lipsticks tested just after being formulated reveal properties characteristic for flow area (G″>G0 ), and those tested
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Influence of propolis on the rheology of lipstick Table I Values of Cross model parameters and amount of dissipated energy DE (samples volume 0.4 9 106 m3) Week of storage
Lipstick without propolis 0
4
Lipstick with propolis 0
4
T, °C
s, s
m
DE 3 103 mJ
27 32 37 42 27 32 37 42
6.2 9.5 15.5 25.1 1.25 1.5 3.15 4.5
1.2
84.4 61.7 22.5 13.8 61.2 65.1 28.5 25.3
27 32 37 42 27 32 37 42
7.6 15 25 25 2 1.2 2.5 4.1
1.2
highlights the effect of the addition of propolis. The values of aT coefficient for lipsticks with propolis (Fig. 3) are considerably higher, which indicates a greater differentiation of apparent viscosity as a function of temperature. Textural properties
1.5
1.5
41.9 39.8 14.8 9.9 44.6 64.9 34.5 17.5
after 4 weeks exhibit different properties. G0 and G″ values become equal for temperatures of 32 and 37°C and G0 >G″ for temperatures of 27 and 42°C. Temperature scaling of apparent viscosities of the lipsticks reveals that both propolis addition and storage period affect the rheological properties within the flow area. For lipsticks with propolis, the apparent viscosity requires scaling with the use of aT factors with lower values than aT factors obtained for lipsticks without the addition of propolis. This means that the effect of temperature on their apparent viscosity is weaker, in comparison with the lipsticks without propolis. For both types of lipsticks, the 4-week storage period changes the values of the scaling factors aT and
The next figure (Fig. 5) illustrates the results of the textural tests of lipsticks with and without (control) propolis, performed at 20°C on the day of preparation. Additionally, we tested the properties of a lipstick with propolis, stored for approximately 5 h at a temperature of 32°C. The lipstick without propolis is characterized by the lowest fragility (4.2 Pa) and the maximum hardness (5 Pa). At the same time, it is the most elastic, as crushing occurs only after reducing the distance of the sensor to h = 11.8 mm. It means that under these conditions, the lipstick is deformed without causing cracks. For the lipstick with propolis, the crushing occurs at h = 12.8 mm; this requires the use of forces in the range of 2.5–3 Pa, which is slightly lower than in the control lipstick. This may indicate that the lipsticks with propolis have limited elastic properties and can crack more easily. Further operation of the sensor leads to the crushing of the lipsticks, and the obtained crushing force values vary for the tested lipsticks and are as follows: for the control lipstick the crushing force value is 5 Pa, 4.5 Pa for lipsticks at the temperature of 20°C and 3.8 Pa for the lipstick heated to the temperature of 32°C. The differences in the properties of lipsticks with propolis, stored at 20 and 32°C, are insignificant. Conclusions The rheological characteristics of lipsticks provide valuable information about the product’s usage parameters, for example contact time with the skin, where the lipstick was applied. They are described by a number of parameters, amongst which yield stress and apparent viscosity are the most important. A high yield stress
Figure 5 Graphical representation of textural tests of lipsticks. Solid line represents decreasing sensor distance as a function of time.
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value may cause difficulty in the distribution of the product onto the skin. For the tested lipsticks, the obtained values of s0 are highest at 27°C, and they decrease as the temperature rises. Four-week storage period of the lipsticks affects the s0 values at lower temperatures. From the application point of view, neither temperature increase nor 4-week storage time causes degradation of the lipsticks’ properties. This allows us to assume that temperature changes occurring during lipsticks’ storage in consumer conditions will not cause mechanical damage to the product. Another important rheological parameter, from the application point of view, is the apparent viscosity. A high value of the apparent viscosity ensures product’s longer contact time with its place of application, which is beneficial in the case of lipsticks. The extended contact time with the skin is an advantage, because the lipstick forms an occlusive protective layer on the skin. The temperature conditions on the skin’s surface (higher temperature) require the creation of a product, which would be characterized not only by high apparent viscosity, but also by appropriate thixotropic properties. Propolis
addition to the tested lipsticks reduces the hysteresis effect of the flow curves (particularly at higher temperatures). The properties characteristic of shear-thinning systems ensures the appropriate application of the lipstick onto the skin. The reduction of the thixotropy effect, by addition of propolis to the lipstick, allows for better reconstruction of the cosmetic’s structure after the shear forces are removed – that is after the lipstick has been applied to the skin. For this reason, the addition of propolis has beneficial effects on application properties of the studied lipsticks. The analysis of textural properties of the lipsticks indicates that lipsticks with propolis are more brittle and prone to crushing. However, the temperature increase (30°C) does not cause significant changes in the textural characteristics of these lipsticks. Acknowledgement Research funded by grants for statutory activities for the year 2014 granted by Polish Ministry of Science and Higher Education.
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