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Home , Cream (pharmacy), Lanolin, Petroleum jelly

Korea-Australia Rheology Journal Vol. 22, No. 4, December 2010 pp. 279-289

Rheological evaluation of jelly as a base material in ointment and formulations with respect to rubbing onto the

Eun-Kyoung Park and Ki-Won Song* Department of Organic Material Science and , School of Chemical Engineering, Pusan National University, Pusan 609-735, Korea (Received July 26, 2010; final revision received November 15, 2010; accepted November 22, 2010)

Abstract

The objective of the present study is to systematically characterize a nonlinear viscoelastic behavior of in large amplitude oscillatory shear flow fields correspondent to the rubbing condition onto the human body. With this aim, using a strain-controlled rheometer, the dynamic viscoelastic properties of commercially available petroleum jelly have been measured at 37oC (body temperature) over a wide range of strain amplitudes at several fixed angular frequencies. In this article, the strain amplitude dependence of the dynamic viscoelastic behavior was firstly reported in detail from the experimentally obtained data and then the results were explained from a structural view-point of petroleum jelly. Nextly, a comparison of elas- tic and viscous properties was made in small and large strain amplitude ranges and then these results were discussed in depth with a special emphasis on their importance in actual usage situations (i.e., rubbing onto the human body or ). Main findings obtained from this study can be summarized as follows : (1) Both the storage modulus and loss modulus show a linear behavior only within an extremely small strain ampli- tude range (γ0 <γ0.2 % ) and exhibit almost an equivalent strain limit of linear response (EL ≈≈γVL 0.2 % ). (2) Both the storage modulus and loss modulus demonstrate a qualitatively similar strain-dependent nonlinear behavior (i.e., strain-thinning feature), even though the storage modulus shows a stronger dependence on strain amplitude than does the loss modulus. (3) As the strain amplitude is increased, the difference between the storage modulus and loss modulus is gradually decreased and subsequently a viscous property becomes superior to an elastic property at sufficiently large strain amplitude range. (4) A large amplitude oscillatory shear flow behavior can provide a plentiful information for a better understanding of the complicated rheo- logical behavior of semi- ointment-like materials in their actual application onto the human body or skin. Keywords: petroleum jelly, rheology, large amplitude oscillatory shear flow behavior, nonliear viscoelastic behavior, storage modulus, loss modulus, strain-thinning behavior

1. Introduction in the study of semi-solid systems (Colo et al., 2004) which are widely found in everyday human life (e.g., oint- In recent years, pharmaceutical and cosmetic industries ments, creams, , pastes and ) and whose rheo- in advanced countries have recognized the importance of logical properties become directly related to the rheological assessment of raw materials as well as final consumer’s requirements. products. With an advancement of computer simulation Furthermore, rheological studies of these semi-solid techniques, it has become possible to estimate the stress- materials are of great significance not only to suggest the strain relationships encountered during , methods for quality control during and/or after manufac- packaging, distribution, storage and application (Lee et al., turing processes but also to provide knowledge for pre- 2008). paring formulations or assessing the texture of the final Rheological study has also been proved to be greatly products (Herh et al., 1998). However, most of semi-solid helpful with respect to product characterization and pharmaceutical and cosmetic systems demonstrate a very improvement. The most important application of rheolog- complicated rheological behavior that is difficult to char- ical knowledge in pharmaceutical and cosmetic fields lies acterize by means of the traditionally-used methods. In general, rheological knowledge is required in a wide variety of pharmaceutical and cosmetic areas including *Corresponding author: [email protected] (Davis, 1971) : (a) quality control of semi-solid products ; © 2010 by The Korean Society of Rheology

Korea-Australia Rheology Journal December 2010 Vol. 22, No. 4 279 Eun-Kyoung Park and Ki-Won Song

(b) storage stability of semi-solid products ; (c) correlation From our previous study (Park and Song, 2010) that of physical parameters with sensory assessment and con- dealt with a nonlinear rheological behavior in steady shear sumer evaluation ; (d) effects of consistency on the per- flow fields correspondent to the spreading condition onto cutaneous absorption of drugs ; (e) effects of formulation the human body, it was found that : (a) petroleum jelly on consistency (e.g., action of self–bodying agents) ; (f) exhibits a finite magnitude of yield stress which is attrib- prediction of flow behavior under the shear deformation uted to its three-dimensional network structure that can conditions encountered in manufacture (e.g., pumping, show a resistance to flow ; and (b) petroleum jelly dem- milling and packaging). onstrates a pronounced non-Newtonian shear-thinning flow In , petroleum jelly (also called behavior which is well described by a power-law equation petrolatum or ) is mainly used as a base material in and may be interpreted by the disruption of a crystalline formulating ointments and creams (dermatological prep- network under the influence of mechanical shear defor- arations). Because of its highly lipophilic character, petro- mation. leum jelly is also used as an essential ingredient in the These informations are greatly valuable in the sense that formulations of cosmetic products. It is further used as a the existence of a yield stress plays an important role in masking ointment and as a base for hydrophilic systems determining a storage stability and a sensory feature of the containing emulsifiers. Such a widespread usefulness of product and that a shear-thinning behavior enhances sen- petroleum jelly is primarily due to its excellent ability in sory qualities of pharmaceutical and cosmetic products in providing lubricity and moisture resistance to various kinds which petroleum jelly is used as a base material during of semi-solid pharmaceutical and cosmetic products such their actual usage (i.e., spreading onto the human body) as ointments, creams, lotions and hand cleaners. (Park and Song, 2010). Petroleum jelly is chemically related to mineral . However, steady shear flow properties alone are not able While contains mainly at to provide a sufficient information when considering an room temperature, petroleum jelly is a mixture of solid and actual product application condition such as rubbing a liquid hydrocarbons and maintains a solid-like state at pharmaceutical ointment or a cosmetic cream onto the room temperature. Petroleum jelly may thus be considered human body or skin. This is because it is easily realized to be a soft-type microcrystalline with a high oil con- that most rubbing processes can reasonably be assumed to tent (Pena et al., 1994). be in nature a periodically oscillatory motion with large Since petroleum jelly is a major ingredient in a wide strain (or stress) amplitudes and consequently the materials variety of topical ointment and cream formulations, the show a nonlinear behavior in their responses. quality and function of these products are greatly con- In order to characterize a rheological behavior of phar- trolled by the rheological (or mechanical) properties of maceutical and cosmetic products in actual application petroleum jelly itself. Hence, through a systematic char- condition (i.e., rubbing onto the human body), therefore, acterization and a complete understanding of the funda- large amplitude oscillatory shear (LAOS) flow measure- mental nature of petroleum jelly, better decisions can be ment becomes far more realistic than steady shear flow made as to the choice of a specific grade of petroleum jelly rheometry. When conducting LAOS flow measurement by for a particular product and for a subsequent manufacture the use of a rotational rheometer, the so-called strain (or of the final product (Fu and Lidgate, 1985). stress)-sweep test must be preferentially performed at a As was briefly mentioned in our previous article (Park fixed angular frequency with increasing the amplitude of and Song, 2010), many attempts have been made to inves- the imposed strain (or stress) for the purpose of deter- tigate the rheological properties of petroleum jelly during the linear viscoelastic region as well as investi- the past several decades by means of a continuous shear gating the nonlinear viscoelastic behavior at large shear viscometry, creep/creep recovery tests, and small ampli- deformations. In addition, in LAOS flow experiment, the tude oscillatory shear measurements (Boylan, 1966 ; human body (or skin) and hand surfaces can be idealized Davis, 1969 ; Fu and Lidgate, 1985 ; Pena et al., 1994 ; in the form of circular parallel plates and the gap size Pandey and Ewing, 2008). However, only a little attention between the plates can be altered to cover a wide range of has been given to the rheological characterization in actual actually available usage conditions. usage conditions such as spreading and rubbing onto the Based upon the above-described backgrounds, as a sec- human body or skin, even though such a rheological infor- ond step of our serial works, the objective of the present mation is much more essential from a view-point of con- study is to systematically characterize a nonlinear vis- sumer’s demands (Lee et al., 2008 ; Cha et al., 2009). This coelastic behavior of petroleum jelly in large amplitude is a main motivation that we have designed a compre- oscillatory shear flow fields correspondent to the rubbing hensive study as to the overall rheological evaluation of condition onto the human body. With this aim, using a petroleum jelly in a wide variety of flow fields most rel- strain-controlled rheometer, the dynamic viscoelastic prop- evant to its actual application conditions. erties of commercially available petroleum jelly have been

280 Korea-Australia Rheology Journal Rheological evaluation of petroleum jelly as a base material in ointment and cream...... measured at 37oC (body temperature) over a wide range of sample in response to the imposed shear strain. When strain amplitudes at several fixed angular frequencies. operating this instrument, the dynamic/steady shear strain In this article, the strain amplitude dependence of the is applied by the step-motor and the torque value is mea- dynamic viscoelastic behavior was firstly reported in detail sured by the force rebalance transducer (FRT). from the experimental data obtained from strain-sweep In this study, in order not only to determine the linear vis- tests and then the results were explained from a structural coelastic region but also to investigate the nonlinear vis- view-point of petroleum jelly. Nextly, a comparison of coelastic behavior in large amplitude oscillatory shear flow elastic and viscous properties was made in small and large fields, dynamic strain-sweep measurements were per- strain amplitude ranges and then these results were dis- formed for petroleum jelly sample using an ARES cussed in depth with a special emphasis on their impor- equipped with a parallel-plate fixture with a radius of tance in actual usage situations (i.e., rubbing onto the 25 mm and a gap size of 2.5 mm. These strain-sweep tests human body or skin). In addition, a distinction was made were carried out at an isothermal condition of 37oC over a between strong and weak gels for petroleum jelly by exam- strain amplitude range from 0.025 to 250% with a loga- ining the strain amplitude dependence of the complex rithmically increasing scale at several fixed angular fre- modulus and finally the future studies planned to be per- quencies of 0.05, 0.1, 1, 5, 10 and 50 rad/s. formed were briefly announced. The reasons why a parallel-plate configuration was cho- sen as a test geometry are that (Song et al., 2006) : (1) 2. Experimental Details cleaning is very easy after each measurement ; (2) the plates can be easily covered with sandpaper ; and (3) there 2.1. Materials is a relatively smaller gap error due to a larger gap size The petroleum jelly sample used in this study was a com- between the two plates (2.5 mm in this experiment) com- mercially available product (White Petrolatum USP) sup- pared to a cone-plate fixture where the gap at the center is plied from the Vi-Jon Company (St. Louis, MO, USA). usually kept at 0.05 mm. This sample is a translucent and unctuous substance having Before the petroleum jelly sample was loaded, the two almost no odor or taste. It is derived from the refinement plates were covered with sandpaper in order to remove a of crude petroleum oil and is consequently a complex mix- wall slippage between the test material and the plates. ture of straight chain, branched chain and cyclic hydro- Through a preliminary test using a direct visualization carbons with varying chain lengths. technique (Chang et al., 2003) in which a straight line It is known that petroleum jelly consists of both solid and marker was drawn from the upper plate to the lower plate liquid hydrocarbons (normal, iso and ring paraffins) in the passing through the free surface of the sample, it was con- form of a structure, thus maintaining a solid-like state firmed that a wall slip effect could almost be eliminated at room temperature (Birdwell and Jessen, 1966 ; Barry over a strain amplitude range tested by covering the plate and Grace, 1971). This gel structure is composed of a surfaces with sandpaper. three-dimensional crystalline network which encloses and Special care was taken to minimize the effect of work immobilizes the liquid hydrocarbons. Disruption of a net- softening when the petroleum jelly sample was initially work structure causes a liquid separation of petroleum loaded on the plate each time. The sample filled up the jelly, after then imparting a flow-ability to this material. whole gap by lowering the upper plate down to the pre- Since petroleum jelly is a major ingredient in a wide designed gap. The extra sample around the edge of the variety of topical ointment and cream formulations, the plate was trimmed with a plastic spatula. quality and function of these formulations are dominantly In all measurements, a fresh sample was used and rested dependent on the mechanical and physico-chemical prop- for 40 min after loading to allow material relaxation and erties of petroleum jelly. In addition, because most of top- temperature equilibration. It was found from a preliminary ical ointment and cream formulations require a dispersion test that 40 min of resting time is enough for petroleum of an internal phase into petroleum jelly, a great deal of jelly sample to be completely relaxed and to be thermally mixing or mechanical shear is needed to achieve a desired equilibrated. All measurements were made at least three homogeneity of the final products. times for each test and highly reproducible data were obtained within the coefficients of variation of ±5% in all 2.2. Rheological measurements cases. The rheological measurements were conducted using an Advanced Rheometric Expansion System (ARES) [Rheo- 3. Results and Discussion metric Scientific, Piscataway, NJ, USA]. ARES is a strain- controlled rheometer that is capable of subjecting a test 3.1. Nonlinear viscoelastic behavior material to either a dynamic or a steady shear strain and Fig. 1(a) shows the strain amplitude dependence of the then measuring the resultant torque values expended by the storage modulus, G′()γ0 , at several fixed angular frequen-

Korea-Australia Rheology Journal December 2010 Vol. 22, No. 4 281 Eun-Kyoung Park and Ki-Won Song

Fig. 1. (a) Storage modulus and (b) reduced storage modulus as Fig. 2. (a) Loss modulus and (b) reduced loss modulus as a func- a function of strain amplitude for petroleum jelly at 37oC tion of strain amplitude for petroleum jelly at 37oC with with different fixed angular frequencies. different fixed angular frequencies. cies (ω = 0.05, 0.1, 1, 5, 10 and 50 rad/s) for petroleum extremely small strain limit of linear elastic response and jelly at 37oC. As is obvious from Fig. 1(a), the storage then demonstrates a pronounced strain-thinning elastic fea- modulus is gradually increased as an increase in angular ture in nonlinear region. Such a trend is consistently seen frequency at relatively smaller range of strain amplitudes at all fixed angular frequencies and more dramatically (γ0 < 1%). This is in accordance with the result obtained manifested when the reduced storage modulus [a ratio of from dynamic frequency-sweep measurement (Park, 2008), the strain-dependent storage modulus, G′()γ0 , to the strain- which will be reported in our forthcoming article. independent storage modulus, G′()0 ] is plotted as a func- A more important point to be noted is that, at all applied tion of strain amplitude, as displayed in Fig. 1(b). angular frequencies, a linear behavior is observed only Fig. 2(a) shows the strain amplitude dependence of the within an extremely small strain amplitude range (γ0 < loss modulus, G″()γ0 , at several fixed angular frequencies 0.2%) in which the storage modulus exhibits a constant (ω = 0.05, 0.1, 1, 5, 10 and 50 rad/s) for petroleum jelly at value regardless of strain amplitude. After then, a nonlinear 37oC. Likewise the storage modulus [Fig. 1(a)], the loss behavior occurs at strain amplitude range larger than 0.2 % modulus is progressively increased as an increase in angular where the storage modulus is slightly decreased up to a frequency at relatively smaller range of strain amplitudes certain strain amplitude (γ0 ≈1 % ), beyond which a sharp (γ0 < 1%). This is also in agreement with the result obtained decrease in storage modulus is observed with increasing from dynamic frequency-sweep measurement (Park, 2008), strain amplitude. which will be reported in our forthcoming article. These results indicate that petroleum jelly has an As is similar to the storage modulus [Fig. 1(a)], at all

282 Korea-Australia Rheology Journal Rheological evaluation of petroleum jelly as a base material in ointment and cream...... applied angular frequencies, a linear behavior is firstly destruction and formation of its internal structure respond- observed only within an extremely small strain amplitude ing to an externally imposed stimulus. range (γ0 < 0.2%) in which the loss modulus maintains a Petroleum jelly forms a gel-like structure comprised of constant value regardless of strain amplitude. Nextly, a both solid and liquid hydrocarbons (normal, iso and cyclic nonlinear behavior takes place at strain amplitude range paraffins) attached through random entanglement and larger than 0.2% where the loss modulus is slowly chemical bonding (Birdwell and Jessen, 1966). This struc- decreased up to a certain strain amplitude (γ0 ≈1 % ), ture is composed of a three-dimensional crystalline net- beyond which followed by a rapid decrease in loss mod- work which encloses and immobilizes the liquid ulus with increasing strain amplitude. hydrocarbons. When subjected to mechanical shear defor- These results imply that petroleum jelly has an extremely mation, a crystalline network becomes disrupted, causing a small strain limit of linear viscous response and then dem- liquid separation of petroleum jelly. onstrates a marked strain-thinning viscous character in At very small strain amplitudes, both the entanglement nonlinear region. Such a tendency is consistently observed density and bonding strength remain unchanged because a at all fixed angular frequencies and more obviously con- balanced state is maintained between the rates of structural firmed when plotting the reduced loss modulus [a ratio of breakdown and rebuilding. Hence, a viscoelastic response the strain-dependent loss modulus, G″()γ0 , to the strain- to an externally applied deformation exhibits a linear independent loss modulus, G″()0 ] as a function of strain behavior, resulting in a constant magnitude of storage mod- amplitude, as illustrated in Fig. 2(b). ulus as well as loss modulus. From a closer examination of Fig. 1(b) and Fig. 2(b), it can As the strain amplitude is increased, however, since the be summarized that : (1) both the storage modulus and loss rate of structural destruction is gradually increased whereas modulus show a linear behavior only within an extremely that of structural reformation is decreased or eliminated, an small strain amplitude range and exhibit almost an equiv- equilibrium between the two rates becomes destroyed, alent strain limit of linear response (γEL ≈≈γVL 0.2 % ) ; and leading to a reduction in entanglement density and bonding (2) both the storage modulus and loss modulus demon- strength. As a consequence, a viscoelastic response to an strate a qualitatively similar strain-dependent nonlinear externally applied deformation exhibits a nonlinear behav- behavior (i.e., strain-thinning feature) with an increase in ior, resulting in a sharp decrease in storage modulus and strain amplitude, even though the storage modulus shows a loss modulus. stronger dependence on strain amplitude than does the loss modulus. 3.2. Comparison of elastic and viscous properties First of all, such a small strain limit of linear response Fig. 3(a) ~ (d) represent both the storage modulus, observed in this work has rarely been reported in the lit- G′()γ0 , and loss modulus, G″()γ0 , as a function of strain erature for other complex materials. According to our pre- amplitude at several fixed angular frequencies of 0.05, 0.1, vious studies, for instances, other viscoelastic materials 1 and 5 rad/s, respectively, for petroleum jelly at 37oC. As maintain their linear behavior up to the strain amplitudes of is clear from Fig. 3(a) ~ (d), the storage modulus is always approximately 40% for ordinary polymeric such as greater than the loss modulus within a linear viscoelastic polyethylene oxide and polyisobutylene (Song et region, indicating that the rheological behavior of petro- al., 1996 ; Song and Chang, 1998 ; Chang, 2010), 10% for leum jelly in this region is in nature dominated by an elas- natural polysaccharide systems such as concentrated xan- tic property rather than a viscous property. In addition, both than gum in aqueous media (Song et al., 2006 ; Kuk, the storage modulus and loss modulus begin to show a 2008), and 5% for oil-in-water food systems such nonlinear behavior at almost a same strain amplitude. as commercial mayonnaises (Ahn and Song, 2008 ; Ahn, As the strain amplitude is increased, however, the dif- 2010), respectively. ference between the two moduli is gradually decreased, Such an extreme shear sensitivity observed for petroleum and subsequently a viscous property becomes superior to jelly may be attributed to its unstable three-dimensional an elastic property at sufficiently large strain amplitude network structure that shows a weak resistance to an exter- range because the storage modulus demonstrates a stronger nally applied shear deformation. When the strain amplitude strain dependence than does the loss modulus. larger than a strain limit is imposed, this structure becomes These trends are consistently observed at all fixed angu- easily broken down, and subsequently structural reorien- lar frequencies and can be more interestingly manifested in tation (or rearrangement) takes place with an increase in an another by plotting the phase angle, δγ()0 , as a strain amplitude, leading to a nonlinear viscoelastic behav- function of strain amplitude, as illustrated in Fig. 4. ior at large strain amplitudes. As can be seen from Fig. 4, with an increase in strain Nextly, a remarkable strain-thinning behavior of storage amplitude, the phase angle is slowly increased up to a cer- modulus and loss modulus for petroleum jelly in nonlinear tain strain amplitude, beyond which a sharp rise in phase viscoelastic region can be interpreted as follows by the angle is observed at all imposed angular frequencies. Ulti-

Korea-Australia Rheology Journal December 2010 Vol. 22, No. 4 283 Eun-Kyoung Park and Ki-Won Song

Fig. 3. Storage modulus and loss modulus as a function of stain amplitude for petroleum jelly at 37oC at a fixed angular frequency of (a) 0.05 rad/s, (b) 0.1 rad/s, (c) 1 rad/s, and (d) 5 rad/s.

though the strain amplitude value at which an elastic prop- erty and a viscous property is reversed is gradually decreased as an increase in imposed angular frequency. Through a thorough investigation as to the nonlinear vis- coelastic behavior of different kinds of complex materials, Hyun et al. (2002 ; 2003) found that there exist at least four types of large amplitude oscillatory shear flow behavior : as an increase in strain amplitude, (a) Type I [(strain-thin- ning) both the storage and loss moduli are decreased] ; (b) Type II [(strain-hardening) both the storage and loss mod- uli are increased] ; (c) Type III [(weak strain-overshoot) the storage modulus is decreased whereas the loss modulus is first increased and then decreased] ; and (d) Type IV [(strong strain-overshoot) both the storage and loss moduli are first increased and then decreased]. These authors also Fig. 4. Phase angle as a function of strain amplitude for petroleum o suggested that large amplitude oscillatory shear flow jelly at 37 C with different fixed angular frequencies. behavior can be used as an effective reference for clas- sifying complex structured fluids. According to their clas- mately, the phase angle becomes greater than 45o at suf- sification, a nonlinear viscoelastic behavior of petroleum ficiently large strain amplitude range, implying that a jelly at large strain amplitudes belongs to Type I (strain- viscous nature becomes dominant to an elastic nature, even thinning).

284 Korea-Australia Rheology Journal Rheological evaluation of petroleum jelly as a base material in ointment and cream......

This type of nonlinear viscoelastic behavior has previ- at all fixed angular frequencies, the storage modulus is ously been observed for other complex materials such as always greater than the loss modulus within a linear vis- ordinary polymer solutions (Song et al., 1996 ; Song and coelastic region, indicating that petroleum jelly shows a Chang, 1998 ; Chang, 2010), anhydrous (Park and desirable linear viscoelastic behavior with respect to the Song, 2007 ; Park, 2008), semi-solid pharmaceutical oint- consumer’s requirements. ments (Cha et al., 2009) and personal care products (Kwon On the other hand, during rubbing on the skin, the prod- et al., 2009). uct should smoothly flow ahead. This implies that the rheo- Now it may be meaningful to discuss the importance of logical behavior at large strain amplitude range must be our results with respect to actual usage situations (i.e., rub- governed by a viscous property rather than an elastic prop- bing onto the human body or skin). An ease of application erty. As is obvious from Fig. 3(a) ~ (d), at all fixed angular of semi-solid pharmaceutical and cosmetic products to the frequencies, with an increase in strain amplitude, the dif- surface of the is a significant factor for con- ference between the storage modulus and loss modulus is sumer’s acceptance. This is governed by the texture profile progressively decreased and subsequently a viscous behav- such as extrudability from a container (or a tube), initial ior becomes superior to an elastic behavior at sufficiently sensation upon contact with the skin, spreading ability on large strain amplitude range, also indicating that petroleum the skin, adhesiveness and residual greasiness after appli- jelly fulfills a desirable rheological requirement during an cation (Brummer and Godersky, 1999 ; Brummer, 2006). actual rubbing process. The topical application procedure may be subdivided From the above-discussed facts, it can be understood that into four steps : (1) removal of a product from a container, the most important rheological informations that should be (2) primary (or initial) sensation on the skin, (3) secondary evaluated when considering a rubbing process of phar- sensation during spreading or rubbing on the skin, and (4) maceutical and cosmetic products are the linear and non- final impression due to a residue on the skin. Among these linear viscoelastic behaviors over a wide range of strain steps, the textural properties corresponding to the steps (2) amplitudes. This process can readily be generated using a and (3) are consistency and spreadability, respectively. strain-sweep test program built in a commercially available As was discussed in our previous study (Park and Song, rotational rheometer. The storage modulus and loss mod- 2010), consistency is directly correlated with a yield stress ulus obtained using this instrument are then to become an while spreadability is deeply associated with a shear-thin- indication of a linear viscoelastic behavior in small ampli- ning flow behavior. When considering only the spreading tude oscillatory shear flow fields and a nonlinear vis- process of an ointment-like material onto the human skin, coelastic behavior in large amplitude oscillatory shear flow therefore, the magnitude of a yield stress and a shear-thin- fields, respectively. The present work has been designed ning flow behavior can be regarded as the most significant based upon these backgrounds and thus obtained results rheological properties to fulfill the consumer’s require- are believed to provide a valuable information as to an ments. actual application of petroleum jelly as well as petroleum However, when applying a pharmaceutical ointment or a jelly-based formulations to the human body or skin. cosmetic cream onto the human body or skin, most persons naturally conduct a rubbing action with different strokes 3.3. Evidence of a weak gel system and future and frequencies rather than a spreading action with dif- studies ferent shear rates. In addition, most rubbing processes can Now it may be worthwhile to mention that a nonlinear be reasonably assumed to be a periodically-repeated oscil- viscoelastic behavior in large amplitude oscillatory shear latory motion with large strain amplitudes. For these rea- flow fields enables a better distinction to be made between sons, informations obtained from large amplitude strong and weak gels because deformation dependence of oscillatory shear flow measurements become much more these two classes of materials has been reported to be quite helpful in order to better understand a rheological behavior different (Ross-Murphy and Shatwell, 1993 ; Ross-Mur- of ointment-like materials in their actual usage situations. phy, 1995). Upon the basis of these backgrounds, the rheological At large deformations, strong gels rupture and fail just properties corresponding to the steps (2) and (3) are then to like a solid while weak gels flow without fracture and be the linear viscoelastic behavior at rest or at very small show a recovery of solid-like character provided that a suf- strain amplitudes and the nonlinear viscoelastic behavior at ficiently long time is given, exhibiting a structured fluid- large strain amplitude range, respectively. like behavior. This statement means that, at large strain At an initial stage upon contact with the human skin, it is amplitudes, the complex modulus for strong gels is essen- undesirable for the product to flow down from the skin. This tially strain-independent (linearly viscoelastic) whereas the means that the rheological behavior at rest or at very small complex modulus for weak gels becomes strain-dependent. strain amplitudes must be dominated by an elastic nature Fig. 5(a) and (b) demonstrate the complex modulus, ∗ ∗ ∗ rather than a viscous nature. As is clear from Fig. 3(a) ~ (d), G ()γ0 , and the reduced complex modulus, G ()γ0 ⁄G ()0 ,

Korea-Australia Rheology Journal December 2010 Vol. 22, No. 4 285 Eun-Kyoung Park and Ki-Won Song

the stress output of a viscoelastic material becomes no longer sinusoidal and consequently the stress-strain rela- tionship cannot be described in terms of the strain-inde- pendent storage and loss moduli due to higher harmonic contributions (Dealy and Wissbrun, 1990). In order to analyze these higher harmonic contributions, a group of researchers (Wilhelm et al., 1998 ; 1999 ; 2000 ; Wilhelm, 2002) has developed a Fourier transform rhe- ology that decomposes the stress data in time domain into a frequency-dependent spectrum. Our group has also intro- duced a Fourier transform analysis to interpret a large amplitude oscillatory shear flow behavior of different kinds of complex materials including polymer solutions (Song and Chang, 1998, Chang and Song, 2000 ; Chang, 2010), natural polysaccharide systems (Kuk et al., 2006 ; 2007, Kuk, 2008), O/W emulsion food systems (Ahn and Song, 2009 ; Ahn, 2010). A Fourier transform analysis on the nonlinear viscoelastic behavior of petroleum jelly in large amplitude oscillatory shear flow fields is now in progress in our laboratory and the results will be reported in the near future. However, Fourier transform rheology has a disadvantage in the sense that the variables determined from Fourier transform are difficult to be connected directly with the well-known functions of their linear counterparts such as storage modulus and loss modulus even though it can treat the obtained data in a quantitative manner. A stress decom- position analysis method developed by Cho et al. (2005) has successfully overcome these problems since it decom- poses the obtained data into an elastic part and a viscous part, respectively, just as the theory of linear viscoelasticity Fig. 5. (a) Complex modulus and (b) reduced complex modulus as o does. a function of strain amplitude for petroleum jelly at 37 C In fact, Fourier transform and stress decomposition can with different fixed angular frequencies. be convertible with each other because these two methods are basically equivalent in nature. Equivalences and dif- respectively, as a function of strain amplitude at several ferences between the two methods have been examined fixed angular frequencies (ω = 0.05, 0.1, 1, 5, 10 and recently by Kim et al. (2006), Ewoldt et al. (2008) and Yu 50 rad/s) for petroleum jelly at 37oC. According to the dis- et al. (2009). Especially, Ewoldt et al. (2008) have suc- tinction described above, it is clear from Fig. 5(a) and (b) ceeded in connecting the two methods through an intro- that petroleum jelly can be classified as a weak gel system duction of the Chebyshev polynomial of the first kind. For since the complex modulus exhibits a marked strain-depen- these reasons, a stress decomposition analysis on the non- dent behavior at all imposed angular frequencies. linear viscoelastic behavior of petroleum jelly in large This ability to flow without fracture is one of the most amplitude oscillatory shear flow fields will also be sched- important differences between typical strong gel systems uled to be made in our laboratory. and petroleum jelly. Such a nonlinear viscoelastic behavior In spite of such a complexity in mathematical treatments, is mainly associated with a tendency to form aggregated it is true that a large amplitude oscillatory shear flow structures which are then broken down under the influence behavior can provide a plentiful information for a better of applied large strain amplitudes. understanding of the nonlinear rheology of complex mate- Finally, it should be noted here that, from a purely the- rials. In addition, large amplitude oscillatory shear flow oretical point of view, both the storage modulus, G′()γ0 , measurement has several advantages in that : (a) it allows and loss modulus, G″()γ0 , at large strain amplitude range both the strain amplitude and time scale to be controlled do not possess their mathematical foundations because independently (Yosick et al., 1997) ; and (b) it is a simple these two moduli are defined only within the linear vis- extension of the dynamic experiment of linear viscoelas- coelastic region. When subjected to large strain amplitudes, ticity (Cho et al., 2010), which can be readily made by the

286 Korea-Australia Rheology Journal Rheological evaluation of petroleum jelly as a base material in ointment and cream...... use of a commercially available rotational rheometer. modulus. A remarkable strain-thinning behavior of the two Moreover, it is relatively easy to generate flow because this moduli observed for petroleum jelly in nonlinear vis- test does not involve any sudden jump in speed or position coelastic region can be interpreted by the destruction and (Giacomin and Dealy, 1993), and consequently obtained formation of its internal structure responding to an exter- data are highly reproducible as far as experimental con- nally imposed stimulus. ditions are in the capability of a rotational rheometer. (3) At all fixed angular frequencies, the storage modulus is always greater than the loss modulus within a linear vis- 4. Conclusions coelastic region, indicating that the rheological behavior of petroleum jelly in this region is in nature dominated by an Petroleum jelly (petrolatum) is used as a major ingredient elastic property rather than a viscous property. This implies in a wide variety of topical ointment and cream formu- that petroleum jelly shows a desirable linear viscoelastic lations. The quality and function of these formulations are behavior with respect to the consumer’s acceptance therefore greatly dependent on the rheological (or mechan- because it is undesirable for the product to flow down from ical) properties of petroleum jelly itself. However, only a the skin at rest or at very small strain amplitudes. little attention has been given to the rheological investi- (4) As the strain amplitude is increased, the difference gation of petroleum jelly in actual usage conditions such as between the two moduli is gradually decreased and sub- spreading and rubbing onto the human body or skin. sequently a viscous property becomes superior to an elastic The objective of the present study is to systematically property at sufficiently large strain amplitude range. This characterize a nonlinear viscoelastic behavior of petroleum also implies that petroleum jelly fulfills a desirable rheo- jelly in large amplitude oscillatory shear flow fields cor- logical requirement during an actual rubbing process respondent to the rubbing condition onto the human body. because the product should smoothly flow ahead at large With this aim, using a strain-controlled rheometer, the strain amplitudes. dynamic viscoelastic properties of commercially available (5) Petroleum jelly can be classified as a weak gel sys- petroleum jelly have been measured at 37oC (body tem- tem. An ability to flow without fracture is one of the most perature) over a wide range of strain amplitudes at several important differences between typical strong gels and fixed angular frequencies. petroleum jelly. Such a nonlinear viscoelastic behavior is In this article, the strain amplitude dependence of the mainly associated with a tendency to form aggregated dynamic viscoelastic behavior was firstly reported in detail structures which are then broken down under the influence from the experimental data obtained from strain-sweep of applied large strain amplitudes. tests and then the results were explained from a structural (6) A large amplitude oscillatory shear flow behavior can view-point of petroleum jelly. Nextly, a comparison of provide a plentiful information for a better understanding elastic and viscous properties was made in small and large of the complicated rheological behavior of semi-solid oint- strain amplitude ranges and then these results were dis- ment-like materials in their actual application onto the cussed in depth with a special emphasis on their impor- human body or skin. tance in actual usage situations (i.e., rubbing onto the human body or skin). In addition, a distinction was made Acknowledgements between strong and weak gels for petroleum jelly by exam- ining the strain amplitude dependence of the complex This work was supported by the Basic Science Research modulus and finally the future studies planned to be per- Program through the National Research of formed were briefly announced. Main findings obtained Korea (NRF) funded by the Ministry of , Science from this study can be summarized as follows : and Technology (Grant Number : 2010-0011027). The authors (1) At all fixed angular frequencies, both the storage are grateful to the NRF for the financial support provided modulus and loss modulus show a linear behavior only to the present work. within an extremely small strain amplitude range (γ0 < 0.2%) and exhibit almost an equivalent strain limit of lin- References ear response (γEL ≈≈γVL 0.2 % ). Such an extreme shear sen- sitivity observed for petroleum jelly may be attributed to its Ahn, H. J., 2010, Rheological comparison of different types of O/ unstable three-dimensional network structure that shows a W emulsion–based mayonnaises : Normal, non- and weak resistance to an externally applied shear deformation. low-fat, M.E.Thesis, Pusan National University, Pusan, Korea. (2) Both the storage modulus and loss modulus dem- Ahn, H. J. and K. W. Song, 2008, Rheological comparison of dif- onstrate a qualitatively similar strain-dependent nonlinear ferent types of mayonnaises : Normal, non-cholesterol and behavior (i.e., strain-thinning feature) with an increase in low-fat, Theor. Appl. Rheol., 12, 71-75. strain amplitude, even though the storage modulus shows a Ahn, H. J. and K. W. Song, 2009, Nonlinear rheology of different stronger dependence on strain amplitude than does the loss types of mayonnaises in several transient shear flow fields,

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