68 FORMULATING
crystallization and reduce melt and solution viscosity, which facilitates mixing, pumping, and filling operations during manufacturing. They also lower the glass transition
temperature of film-forming materials, preventing brittle or cracking films after solvent evaporation when applicable. In addition, plasticizers enhance low-temperature flexibility of occlusive barriers, helping maintain a soft, non-waxy sensory profile even in cold environments. By reducing the yield stress of emulsions
and anhydrous balms, they contribute to greater sensory elegance, including improved cushion, extended playtime, and reduced drag. Plasticizers further promote deeper and more uniform penetration of emollients into the stratum corneum by decreasing the viscosity, intermolecular cohesion, and surface tension of the oil phase. The two main categories of plasticizers are
primary and secondary. Primary plasticizers interact directly with wax molecules, whereas secondary plasticizers enhance the effectiveness of a primary plasticizer. Primary plasticizers can act via two mechanisms: internal or external plasticization. Internal plasticization involves chemical
modification of the wax molecule or its building blocks before the wax is synthesized. External plasticization, which is the focus of this publication, occurs when plasticizers are added during formulation at elevated temperatures. Esters are among the most commonly used
external plasticizers due to their favourable physical interactions with the high molecular weight molecules that typically constitute waxes. These interactions allow the wax and plasticizer to form a homogeneous physical unit, preventing phase separation. It is critical that the intermolecular interactions
between wax molecules, typically long-chain hydrocarbons or esters and the like, and plasticizer molecules are carefully balanced within an intermediate range. Interactions that are too strong or too weak can compromise the desired macroscopic properties of the final material. This specific interaction between the wax and
plasticizer molecules causes these materials to form a homogenous physical unit, i.e. they do not separate. It is essential to emphasize that the intermolecular interactions between wax and plasticizer molecules must be carefully balanced within an intermediate state, neither excessively strong nor excessively weak, to achieve the desired macroscopic properties in the final material.6 Overly strong interactions, e.g. extensive
hydrogen bonding networks or strong dipole– dipole associations, would lead to a highly cohesive, tightly packed matrix in which segmental mobility of the wax chains is severely restricted. This would result in a rigid, brittle material with reduced flexibility, poor low-temperature performance, and diminished capacity for stress dissipation, properties against effective plasticization. Conversely, interactions that are too weak (limited to sparse or feeble dispersion forces)
PERSONAL CARE MAGAZINE April 2026
TABLE 1: PLASTICIZERS PERFORM MULTIPLE CRITICAL FUNCTIONS IN PERSONAL CARE FORMULATIONS
Plasticizer function
Suppress crystallization & reduce viscosity
Mechanistic effect
Decreases solid formation and eases flow
Lower glass transition temperature Increases flexibility of polymer/ film
Enhance low-temperature flexibility
Reduce yield stress
Promote deeper emollient penetration
Maintains material softness
Lowers resistance in emulsions/ balms
Decreases viscosity, cohesion, and surface tension
would fail to ensure sufficient thermodynamic compatibility between the components. This could lead to phase separation, exudation (bleeding) of the plasticizer, macroscopic inhomogeneity, and loss of long-term performance stability. The optimal scenario involves a combination
of intermolecular forces of differing strengths: Predominant weak, non-specific van der
Waals (London dispersion) forces that are easily disrupted by thermal motion (ambient or processing temperatures). A controlled, moderate number of stronger but
still reversible polar interactions, such as dipole– dipole attractions, induced dipole interactions, or a limited degree of hydrogen bonding (typically 0.5–2 hydrogen bonds per plasticizer molecule, depending on the size of the molecule and the system).7
These stronger interactions should be localized on specific functional moieties of the plasticizer (e.g., ester carbonyls, hydroxyls, ether oxygens, or aromatic rings) that can transiently associate with complementary electron-rich or electron-deficient sites on the wax molecules (oxygen atoms in ester waxes, terminal hydroxyls in fatty alcohols, etc.), hydrogen bonds being a good example. The association must remain dynamic and
reversible. The plasticizer molecules should undergo continuous attachment–detachment cycles. This constant exchange would allow individual
plasticizer molecules to act as temporary ‘spacers’ and lubricants: they momentarily shield chain–chain contacts, reduce frictional drag during segmental motion, and lower the energy barriers for conformational transitions, yet they are readily displaced by neighbouring plasticizer molecules or by thermal fluctuations resulting in a microscopically heterogeneous but macroscopically homogeneous matrix with enhanced free volume and markedly improved flexibility and ductility.8 Consequently, the molecular design
parameters of the plasticizer, molecular weight and architecture (linear vs. branched), polarity and distribution of functional groups, steric hindrance, conformational flexibility, and the precise balance between non-polar alkyl chains and polar head/tail groups, become paramount. Subtle variations in these parameters can
shift the system from an optimally dynamic, compatible state to either an over-stabilized
Outcome/benefit
Facilitates mixing, pumping, and filling
Prevents brittle or cracking films after solvent evaporation
Keeps occlusive barriers soft, non-waxy in cold climates
Improves sensory elegance, cush- ion, playtime, non-drag
Ensures uniform penetration into stratum corneum
rigid network or an incompatible two-phase mixture. Rational selection, often aided by chemistry considerations and experimental validation (DSC, DMA, rheological studies), is therefore indispensable for achieving the delicate equilibrium required for high-performance wax- based formulations.9
The practical benefits of plasticizers in personal care Improved flow properties and processability The incorporation of these additives significantly enhances the rheological profile of the formulation. By reducing the yield stress and apparent viscosity at processing shear rates, the system becomes markedly easier to mix, pump, and fill during manufacturing. This improvement in flowability not only streamlines large-scale production but also facilitates precise dosing in consumer packaging. Additionally, the optimized rheology
contributes to superior emolliency on skin, as the reduced internal friction allows the product to spread more readily and form a thinner, more uniform occlusive layer.
Superior sensory profile and texture The resulting microstructure* imparts a luxuriously smooth, creamy, and velvety tactile experience. This is achieved through the formation of a finely dispersed crystalline network or lamellar gel phase* that minimizes grittiness and maximizes perceived richness. Consumer perception studies consistently
correlate such refined textures with premium quality and enhanced skin-conditioning benefits. *Oleogels, frequently occur in personal care applications, liquids (oils) immobilized by a 3-D network of structuring agents, resulting in unique properties.
Enhanced physical and chemical stability The robust three-dimensional network created by the structuring system effectively immobilizes the liquid oil phase, dramatically reducing the incidence of syneresis, phase separation, and oil bleed over time and across temperature fluctuations. Simultaneously, it inhibits Ostwald ripening and polymorphic transitions in crystalline components, thereby preventing graininess, grittiness, or flaking.
www.personalcaremagazine.com
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