Impact of Polyethylene Glycol molecular weights and ratios on Rheology and Textural Properties of Ointment Bases

Abstract

This study evaluated the impact of different PEG molecular weights and ratios on ointment rheology and application properties, such as spreadability, adhesiveness, and extrusion. Results emphasized the importance of balancing PEG types and proportions to optimize formulation performance, affecting drug delivery and user experience. Texture analyzer and rotational rheometry showed consistent trends, confirming their similarity in characterizing ointment for stable and efficient formulations.

Introduction

Given the multitude of molecular weights of PEGs and their extensive use in pharmaceutical formulations, understanding their properties and effects on drug development is essential for creating stable and effective formulations. This study evaluates the impact of different PEG molecular weights and ratios on the rheology of ointments and their application characteristics, such as spreadability, adhesiveness, and extrusion. It aims to correlate ease of spreading on the skin, adherence after application, and the force required to remove the ointment from packaging with textural / rotational rheological datasets

Materials and Methods

The ointments were made by combining low and high molecular weight (MW) PEGs at a fixed percentage of 20% Propylene Glycol in reference PEG based formulations [1], resulting in 18 experimental combinations (plus replicates).

Table 1: Ratio and types of PEG used in the formulations

Figure 1: (A) Mechanical Stirrer used to prepare formulations at 1250RPM; (B) MARS60 Rheometer used for rheology analyses; (C) TA.XT/PLUS 100 Texture Analyzer used for spreadability, adhesiveness, and extrusion analyses.

Results*

Figure 2: Visual ointment aspect according to different ratio of the same blend of PEG 400 with the PEG3350.

*Note: The graphs presented here are representative of the overall observations. Due to space constraints, it will not be possible to exemplify the entire study conducted.

For the Texture Analyzer, quantitative results from multiple parameters were obtained. For better visualization and understanding of the characteristics, the data were normalized and are displayed in the qualitative graphs below.

Figure 3: Comparative and normalized overview on the textural analysis data set for different PEG combinations (MW and ratio )

%liquid PEG & the MW of solid PEG = Spreadability and extrusion ease. While for firmness, work of shear, consistency, and cohesiveness the opposite effect is observed.
These results reflect PEGs characteristics, for example: longer cohesive polymer chains like PEG 6000 lead to firmer textures while liquid PEGs, such as PEG 400, act as lubricants and may soften textures.
The rheometer results, along with the representative graphs below, show that: MW solid PEG and %liquid PEG increase storage modulus (G’) and Yield Point, while decreasing Compliance (i.e., facilitating deformation – the higher the value, the greater the deformability). Compliance is the inverse of the storage modulus (G′).

Figure 5 : Rotational Rheology output curves for different combinations.

Conclusion

The results highlighted the critical need to balance PEGs in formulations, as this balance influences the drug’s rheology, affecting API delivery and the user’s sensory experience. Additionally, the study revealed trends in texture properties based on PEG type and proportion, as well as a directly equivalent alignment of the information obtained from the rotational rheometer and the texture analyzer. This indicates the possibility of using both methods to gather information related to the rheology of the ointments.

See the full poster on Impact of Polyethylene Glycol molecular weights and ratios on Rheology and Textural Properties of Ointment Bases here

(click the picture to download the poster)

Source: Indovinya, Rafael Caetano Jardim Pinto da Silva Salvato; Hemyle Rangel Rocha Chaves; Maurício Soares Júnior; Beatriz Rodrigues Pinto; Jordana Sakis Sonza (All for the same affiliation: Indorama Ventures – Indovinya), poster: Impact of Polyethylene Glycol molecular weights and ratios on Rheology and Textural Properties of Ointment Bases, IPEC Americas Excipients World 2025

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