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Delamination and wetting behavior of natural hot-melt coating materials
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Hot-Melt Coating Materials: Enhancing Pharmaceutical and Industrial Applications

Hot-melt coating materials improve efficiency and product quality in pharmaceutical and industrial manufacturing. They melt when heated and solidify quickly, forming strong, uniform coatings on various surfaces. As a result, manufacturers reduce production time, lower costs, and avoid using solvents. Furthermore, understanding wetting behavior and delamination is critical to optimize coating performance. For example, CELLETS® 1000 microcrystalline cellulose pellets serve as excellent starter cores, promoting uniform wetting and consistent coating thickness. Consequently, hot-melt coating materials have become a reliable solution for modern manufacturing needs.

Enhancing pharmaceutical and industrial applications by hot-melt coating materials

In the study titled Delamination and Wetting Behavior of Natural Hot-Melt Coating Materials, published in Powder Technology [1], the authors investigated the delamination and wetting behaviors of various natural materials. The research aimed to understand how these materials interact with substrates during the coating process, which is crucial for applications in the pharmaceutical industry. The study utilized laboratory coating experiments and micro-computed tomographic measurements to assess delamination frequency, and a drop shape analyzer to evaluate wetting behavior. Interestingly, the study found no correlation between delamination and wetting behavior, suggesting that other factors may influence delamination in hot-melt coatings.

Among the materials tested, CELLETS® 1000, a type of microcrystalline cellulose (MCC) pellet with a size range between 1000 and 1400 µm, was highlighted for its suitability in hot-melt coating applications. These spherical pellets are known for their chemical inertness, low friability, high sphericity, and smooth surface, making them ideal as starter cores for multiparticulate drug delivery systems. In the context of the study, CELLETS® 1000 demonstrated excellent wetting properties with contact angles ranging from 10° to 18°, which is favorable for uniform coating. However, the study did not find a direct correlation between wetting behavior and delamination, indicating that other factors may play a more significant role in delamination during hot-melt coating processes. Researchers assume that delamination may have resulted from the different thermal expansion coefficients of the carrier particle and the coating material [2]. A change in temperature may have led to thermal stresses and may have promoted spalling or delamination. Subsequent swelling of a hygroscopic carrier material due to moisture could also lead to structural
changes in the coating structure and might cause delamination.

Use of CELLETS® in hot-melt coating processes

The use of CELLETS® in hot-melt coating processes offers several advantages. Their uniform size distribution and smooth surface contribute to consistent coating thickness and quality. Additionally, the chemical inertness of CELLETS® ensures compatibility with a wide range of coating materials, reducing the risk of undesirable interactions. These characteristics make CELLETS® a reliable choice for developing controlled-release formulations and enteric coatings in pharmaceutical applications.

In summary, the study underscores the importance of understanding the delamination and wetting behaviors of natural hot-melt coating materials. While CELLETS® 1000 exhibited favorable wetting properties, the lack of correlation between wetting behavior and delamination suggests that other factors should be considered when selecting materials for hot-melt coating processes. Further research is needed to identify these factors and optimize coating processes for improved product performance.

References

[1] B.M. Wörthmann et al., Powder Technology (404) 2022, 117443; doi: 10.1016/j.powtec.2022.117443.

[2] S. Ebnesajjad, A.H. Landrock, Introduction and adhesion theories, Adhesives Technology, Handbook, 38, Elsevier 2015, pp. 1–18; doi: 10.1016/B978-0-323-35595-7.00001-2.

Understanding Hot-Melt Coating Materials

Hot-melt coating materials are thermoplastic substances that bond effectively to substrates when melted. Their melting point, adhesion properties, and chemical compatibility directly influence coating uniformity and durability. Therefore, selecting the correct material is crucial for minimizing delamination and ensuring product quality. Additionally, their solvent-free nature makes them environmentally friendly and cost-efficient.

Optimizing Coating with CELLETS®

CELLETS® offer significant advantages as starter cores in hot-melt coating processes. Their spherical shape and smooth surface promote uniform wetting and consistent coating thickness. Furthermore, their chemical inertness ensures compatibility with diverse coating materials, reducing the risk of unwanted interactions. Consequently, these MCC spheres support reliable and high-quality coating outcomes in both pharmaceutical and industrial applications.

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Coating uniformity of hot-melt coated particles Figure 2 (pure)

Coating uniformity of hot-melt coated particles

Abstract

Coating uniformity is a critical parameter in coating processes in novel pharmaceutical formulations. Speaking about pellet technology, coating and layering are the main methods for implementing drug functionalities, such as modified release of the active, taste-masking properties and further more. Coating uniformity guaranties not only upholding functionalities of the formulation, but also prevent risks such as dose dumping.

This application note is based on a publication of Wörthmann et al. [1] and focuses on selected aspects which are related to starter cores.

Cellets 1000, magnification 100x

Figure 1: Microscopic image of Cellets® 1000, magnification 100x.

Materials and techniques

Coating was applied on highly spherical starter cores Cellets® 1000 (Figure 1). The pellets have a relatively narrow size distribution with a mean particle size of d­­­­50 = 1197 μm, a standard deviation of σ = 113 μm, and particle density of 1.4 g/cm3. For analyzing the coating uniformity, stearin (54 % stearic acid and palmitic acid) and hydrogenated palm oil were used. For the hot-melt coating experiments a lab-scale Wurster fluidized bed was used. The overspray rate was estimated to 8 % (w/w). Processed particles were analyzed by image analysis (Figure 2) and micro-computed-tomography (μCT) (Figure 3). 2D and 3D software analysis were further conducted for the evaluation of the sphere dimension, layer thickness and coating uniformity.

Figure 2 shows a wax-coated particle, where the coating thickness varies and delamination is clearly visible (Figure 3). Small pores and fractions of the coating layer area are obvious.

Coating uniformity of hot-melt coated particles Figure 1

Figure 2: Images of coated pellets are used for a stepwise evaluation of the particle shell thickness. A: original image; B: segmented coating layer. Further software calculation steps are not shown here.

These undesired artefacts result from imperfect parameters, such as spreading mechanism, temperature fluctuations, viscosity, or drop size.

The coating layer thickness is analyzed for three particles of the same batch (Figure 4) using 5 % (w/w) stearin at a spraying rate of 1 g/min. The layer thickness varies between approximately 2 µm to 30 µm. A mean coating thickness is found between 12 µm and 16 µm.

Coating uniformity of hot-melt coated particles Figure 2

Figure 3: Portion of a micro-computed-tomography image of a wax-coated particle showing.

Coating uniformity of hot-melt coated particles Figure 3

Figure 4: Relative frequency of the coating layer-thickness of three particle shells from the same batch using 5 % (w/w) stearin at a spraying rate of 1 g/min. Mean thicknesses: particle I (blue): 15.5 μm, particle II (red): 12.4 μm, and particle III (grey): 15.6 μm.

In terms of customer safety and of compliance aspects, not only statistical information about the layer thickness are of interest. In case of inhomogeneous layers, taste-masking functionalities or even uncontrolled dose dumping might occur. In this context, a single-particle analysis is mandatory. 3D µCT is a powerful tool, which is complementary to existing methods, such as laser imaging methods, 2D analysis or thickness estimations. The analyzed mean thickness deviates by approximately 13 % among these methods (Figure 5).

Coating uniformity of hot-melt coated particles Figure 4

Figure 5: Mean layer-thicknesses measured using different methods. Relative standard deviation: 13 %.

Summary

Microcrystalline cellulose pellets (Cellets®) are used to study coating uniformity. 3D μCT can be a powerful tool to assess the quality of the final product coating and facilitates the selection of an appropriate combination of core particles and coating material. 3D visualization methods allow a critical single-particle analysis with a resolution of up to 2 µm. Furthermore, the determination of the particle’s uncoated surface area can be specified.

Acknowledgement

Prof. Heiko Briesen, Mario Wörthmann (Technical University Munich) and team are gratefully acknowledged for serving content for this note.

Research was financially supported by the Ministry of Economics and Energy (BMWi) and FEI (Germany) via project AiF 19970 N. Equipment funded by Deutsche Forschungsgemeinschaft (DFG, Germany) 198187031.

References

[1] B.M. Woerthmann, J.A. Lindner, T. Kovacevic, P. Pergam, F. Schmid, H. Briesen, Powder Technology 378 (2021) 51–59

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