Historically, within the cosmetics industry, personal care articles such as gloves, socks and wraps have been used in conjunction with formulations to provide moisturization or to deliver actives to the skin. Traditionally, users apply a formulation to their hands or feet, then wrap the article over the treated regions. Alternatively, users slip on a pre-treated glove or sock, which then transfers the saturated formulation to the skin due to intimate skin contact. Unfortunately, such items typically have been made of a polymeric material, e.g. neoprene rubber, which lacks a clothlike appearance and feel. Oftentimes, such items also do not readily conform to the complex surfaces and contours of a foot or hand, or they cannot adequately hold the formulation, resulting in leakage of the product.
To address these shortcomings, a personal care article was recently developed1 comprising a three-layered laminated elastic composite with a structure that is generally described as a spunbond-film-spunbond (SFS) material. Spunbond is a strong, flexible material manufactured using wood pulp and continuous fibers of polypropylene that are thermally bonded to create a fabric.2 This material enables a biaxial stretch to ensure proper fit and general ease of use.
Specifically, the laminated substrate within the personal care article was made by sandwiching an elastomeric film between two 50% stretched spunbound nonwoven substrates. The elastomeric film layer included 96%w/w olefin elastomer resina and 4% w/w filler compound containing calcium carbonate blended with polypropylene and polypropylene random copolymers. The substrates were bonded with heat at specific points with the film layer in a stretched state and the resulting composite sample was allowed to retract to give a three-dimensional texture. This texture gave the product a clothlike appearance and helped create better contact between the skin and saturated substrate.1
When developing skin care moisturizers for application via nonwoven substrates, formulators must consider the hydrophobic oils, the affinity of those oils to the substrate, the add-on to the substrate, and the stability of the compositions on the substrate. With these considerations, two moisturizing formulations were developed and coated onto the laminated substrate previously described to produce a moisturizing article to soothe and moisturize dry, cracked skin (see Formulas 1a–b). Formula 1a contained a higher level of hydrophobic oils than emulsifying agents, compared with Formula 1b. The weight of the substrate was measured and both formulations were coated at a 50% add-on relative to the weight of the laminated substrate in a manner that ensured uniform distribution.
It was observed that Formula 1a caused the multiple layers of the substrate to separate from one another or delaminate and as a result, Formula1b alone was used for clinical moisturization testing. Further research on Formula 1a components confirmed that isopropyl palmitate and avocado oil were the primary cause for the delamination and drop of tensile strength of the elastomeric substrate, which are described later in this article. However, no additional research was conducted to determine what fundamental properties of these oils corresponded to the changes in the substrate.
Clinical Moisturization Testing
Clinical studies were then conducted to substantiate claims associated with the product since its distinct advantage is that the user can wear it while performing daily activities. Conversely, applying lotion directly on the feet or hands can be a negative aesthetic experience, and if the user is active following direct application, the lotion transfers undesirably to clothing, surfaces, etc. In addition, as the authors have discovered from previous wet wipes-related projects, although humectants are included in formulations transferred to the skin, their clinical success is not guaranteed.
For the clinical evaluation, 53 adult female subjects with visibly dry, cracked skin on their heels, as determined by an expert grader, were selected to evaluate the moisturization properties of the sock product coated with Formula 1b. Subjects were acclimated to a temperature- and humidity-controlled room. Baseline skin capacitance measurements, an indirect measure of skin moisture, were taken with a skin impedence instrumentb on the top of each foot. Following baseline measurements, subjects wore the socks for 15 min total, during which subjects walked for 5 min, then sat for 10 min.
Following removal of the socks, any residual lotion was rubbed into the skin by a study technician. Capacitance measurements were then obtained 30 min and 2 hr from the time the socks were removed from the foot. A similar procedure was used for measuring the moisturization values of hands after glove-wearing.
Figure 1 shows that after 15 min of sock and glove wear, the mean moisturization values were significantly higher, compared with baseline values 30 min (p < 0.0001) and 2 hr (p < 0.0001) after removal. Also, when subjects were asked whether the sock soothed the dry, cracked skin on their heels, 98% of them reported it did (p < 0.0001) while 79% reported the sock left their feet feeling moderately or very soothed.
Both the sock and glove products significantly increased the mean moisturization values at 30 min and 2 hr after the products were removed, relative to baseline (p < 0.0001). Yet, despite the success of the clinical study, the ability to make further improvements in the moisturizing formulation was not possible due to a limited understanding of the oil delamination phenomena that was witnessed in earlier studies.
Understanding Substrate Delamination
Since the spunbond material contains polypropylene fibers that are hydro-phobic in nature, it was hypothesized that a less polar oil phase would have greater affinity for the polypropylene fibers. This affinity would potentially cause greater instability and have a greater impact on the mechanical properties of the substrate. To test this theory, the dielectric constants of 52 liquids and liquid blends were obtained from the literature or experimentally at 25°C using a dielectric constant meterc.3 A representative subset of these compounds was then selected to cover a broad range of dielectric values. These compounds are listed in Table 1.
These select oils were then uniformly applied to the elastomeric substrate at 100% w/w add on and aged at 55°C for one week to accelerate any interaction between the oils and the substrate. Following aging and an overnight equilibration under TAPPI conditions—i.e., conditions set forth by the Technical Association of the Pulp and Paper Industry (TAPPI), which are defined as 50% RH and 22.8°C—six 25 x 127 mm specimens for each sample were subjected to multicycle stress/strain testing using a tensile testerd with a 100 N load cell and appropriate operating software. The multicycle stress/strain test is a two-cycle elongation and recovery test used to measure the characteristics of elastic raw materials and compositions. The deterioration of substrate properties can be measured by this test as a loss of tension or resistance to elongation, or a break within the substrate when extended.4
The load at 30% extension was selected as the critical value as this corresponded to extensions typically observed when the sock product was worn by subjects. At 30% extension, the load of the uncoated laminate structure was measured to be 299 ± 13 g*f/in.4 Coating the laminate structure with water at 100% did lower the extension to 187 ± 13 g*f/in, or 63% of the original load, but this did not significantly impact the overall function of the saturated laminate substrate.4
However, as shown by Figure 2, traditional hydrocarbon-based emollients and humectants obeyed a fairly predictable relationship (R2 of 0.71) that the load at 30% extension significantly decreased as the dielectric constant of the cosmetic oil decreased. Very hydrophobic oils such as mineral oil reduced the inherent load at 30% extension to 26 ± 2 g*f/in (87% of original load), which caused the laminate layers to separate and dissolve, rendering the saturated laminate useless.4
Despite having equivalent dielectric constants, the silicone-based compounds dimethicone and PEG-8 dimethicone retained a significant portion of the original load at 30% extension, having load values equivalent to laminates treated with water. While further work to understand this relationship is required, generally accepted silicone solubility properties could be applied. Having polyolefin-based substrates, hydrocarbon-based compounds such as mineral oil are more likely to strongly interact with and plasticize polymer strands within the nonwoven substrates, reducing the overall strength and leading to lower overall load values. In contrast, a silicone with a similar polarity to its hydrocarbon counterpart did not affect the overall appearance or tensile strength of the substrate.
Further tests to verify this relationship were performed using 12 additional types of elastomeric nonwovens and yielded similar results. These substrates contained various elastomeric resins or combinations of theme.
Optimizing Nonwoven Substrate Formulas
With these results in mind, a simple w/si emulsion was created (see Formula 2) and coated onto the same type of laminate structure as was tested with the individual oil or oil blends and tested in the same manner. Test results are shown in Table 2. The measured dielectric constants of the moisturizing, low-oil content formulation (Formula 1b) and the w/si emulsion (Formula 2) were 44 and 6.8, respectively.5 Despite having more than 10 times the amount of oil, Formula 2 had equivalent load values to both Formula 1b and even the elastomeric nonwoven coated with water.
It was further hypothesized that greater levels of interaction between hydrocarbon oils and the polyolefin-based elastomeric laminate would result in a lower overall formula transfer from the nonwoven web to the skin. In contrast, the lower levels of potential interaction between silicone oils and the polyolefin-based elastomer were expected to transfer greater amounts of the saturated formulation. Also, more polar molecules with higher dielectric constants would be expected to transfer higher amounts of the formulation.
To determine transfer rates of formulations or oils from the substrate, the elastomeric laminate was first cut into a 2-in diameter circle using an appropriately shaped die and pneumatic press.5 The formulation or oil of interest was uniformly applied to the substrate at 100% w/w add-on. To improve statistical significance, six replicate experiments were performed per oil or composition. Following a 12-hr acclimation period under TAPPI conditions, the sample weight of the substrate and infused formula/oil was recorded. The sample was then placed in between six layers paper towelf. A weight sufficient to provide a 0.06 psi load was then distributed across a plexiglass plate placed on top of the paper towel and left for approximately 15 min. Following the load, the sample substrate was weighed and the percent weight loss was calculated. The percentage of weight loss was assumed to be the percent transferred to the paper towel (see Figure 3).5 Readers should note that obviously, different forces of attraction are present when the composition contacts skin; however, the paper towel provided a preliminary assessment of the formulation transfer.
Results and Conclusions
It was anticipated that a lower viscosity fluid would transfer more readily than a higher viscosity fluid. As shown by Figure 3, as the viscosity of the various grades of silicone oil increased, the amount of silicone oil transferred from the nonwoven substrate decreased. However, when one compares the transfer efficiency of the 10 cst dimethicone to isopropyl palmitate and mineral oil, which have equivalent viscosities, a substantial difference is observed.
As seen in Figure 3, both isopropyl palmitate and mineral oil did not transfer readily from the substrate and were found to significantly reduce the load values of elastomeric nonwovens, whereas no significant change was observed with silicone based fluids of a similar viscosity. Further, despite having a significantly lower polarity, dimethicone exhibited transfer efficiency comparable to much more polar materials such as PEG-5 methyl ether and propylene glycol.
Together, these results indicate that future developments for moisturizing formulations intended for delivery from polyolefin elastomer articles should focus on increasing the overall polarity of the oil phase or ensure a significant portion of the oil phase is composed of silicone-based emollients and occlusive agents. These changes would not only ensure the proper function of the elastomeric article is maintained, but also may provide greater clinical benefits via greater transfer efficiencies of the formulation to the skin.
1. US Patent 2007/0026028A1, Appliance for delivering a composition, K Close et al, Kimberly-Clark Corp (Feb 1, 2007)
2. The Chemistry and Manufacture of Cosmetics, vol II, Formulating, 4th ed, ML Schlossman, ed, Allured Publishing Corporation, Carol Stream, IL USA, ch 33 (2009) pp 1093
3. US Patent S7014842B2, Sunscreen composition, OV Dueva-Koganov and JP SaNogueira, Playtex Products Inc (Jan 20, 2005)
4. US Patent 2010/0008957A1, Formulations having improved compatibility with nonwoven substrates, S Mundschau et al (Feb 14, 2010)
5. US Patent 2010/0008958A1, Substrates having formulations with improved transferability, S Mundschau et al (Feb 14, 2010)