Relationships among material plasticity, tablet brittleness, and tensile strength

Abstract

Tablet mechanical strength is governed by both the intrinsic mechanical properties of the constituent materials and the applied compaction conditions. In this work, we investigated the relationships among tablet tensile strength, tablet brittleness, quantified by the tablet brittleness index, and powder plasticity, quantified by in-die mean yield pressure. Seven common excipients and twelve binary mixtures were selected to represent materials spanning a wide range of mechanical behaviors. For a given material, tablets become more brittle and weaker as porosity increases, following an exponential decay relationship. At a fixed tablet porosity, in-die mean yield pressure shows a positive correlation with tablet brittleness index that follows a power-law function. This relationship enables prediction of tablet brittleness index at a specified porosity directly from in-die mean yield pressure. Because in-die mean yield pressure can be readily obtained from in-die compression data using only small quantities of material, it offers an efficient means to estimate tablet brittleness early in development and provides valuable guidance for designing robust tablets.

Introduction

Brittleness is a fundamental mechanical property with significant implications across diverse fields, including pharmaceutical industry (Andreev, 1991, Girardot et al., 2023, Hucka and Das, 1974, Lebihain et al., 2023, Mazel and Tchoreloff, 2023). In general, a brittle pharmaceutical material is characterized by minimal resistance to fracture under applied stress (Robert and Rowe, 1987, Sonnergaard, 2013, Vreeman and Sun, 2022a). This behavior contrasts with that of ductile materials, which can more easily dissipate applied stress prior to crack initiation and fracture (Gong and Sun, 2015, Paul and Sun, 2017, Robert and Rowe, 1987, Sonnergaard, 2013).

Brittleness is typically regarded as an inherent material property (Hucka and Das, 1974, Lebihain et al., 2023), which plays a crucial role in governing tablet deformation and fracture under mechanical stress. Tablet brittleness pertains to immediate-release tablets that need to undergo rapid breakup and disintegration within the gastro-intestinal tract to elicit fast pharmacological action (Chamsai and Sriamornsak, 2013, El Maghraby and Elsergany, 2014, Preis, 2015, Singh, 1992, Wagner and Krumme, 2000). High brittleness is also preferred for chewable tablets. However, tablets with high brittleness are more susceptible to damage during manufacturing, transportation, and handling (Hiestand, 1997, Paul and Sun, 2017). High tablet brittleness has been strongly associated with manufacturing defects, such as capping, lamination, and elevated friability (Gong et al., 2015, Osei-Yeboah and Sun, 2015). Nevertheless, a recent study suggested that the correlation between tablet brittleness and friability is valid only when brittleness is interpreted as “easy to break”, rather than as “elastic until failure” (Mazel and Tchoreloff, 2023). Capping and lamination, which are commonly attributed to air entrapment during compression, tend to weaken the tablet structure and increases both brittleness and friability (Garner et al., 2014, Klinzing and Troup, 2019, Vreeman and Sun, 2022b). As a result, more brittle tablets require higher tensile strength to meet the USP friability criterion (Girardot et al., 2023, Mazel and Tchoreloff, 2023, Mizunaga and Watano, 2025, Osei-Yeboah and Sun, 2015, Sonnergaard, 2023). Thus, maintaining an appropriate balance between tablet brittleness and mechanical strength is essential to achieving tablets that meet desired quality attributes.

Various brittleness indices have been reported in the literature to quantify the brittleness of pharmaceutical solids, including: 1) the brittle fracture index (BFI) (Hiestand et al., 1977, Hiestand and Peot, 1974, Hiestand and Smith, 1984), 2) the brittle ductile index (BDI) (Sonnergaard, 2013), and 3) the tablet brittleness index (TBI) (Gong and Sun, 2015, Paul and Sun, 2017). The BFI was originally proposed to quantify the brittleness of compacted materials, defined by comparing the tensile strength of a tablet with that of an otherwise identical tablet containing a central hole (Hiestand and Peot, 1974). BFI value is 0 for materials exhibiting predominantly plastic deformation under stress and 1 for materials exhibiting complete brittle behavior (Hiestand and Peot, 1974, Hiestand and Smith, 1984). However, negative BFI values have been reported, for example when lactose monohydrate tablets containing a central hole exhibited higher tensile strength than intact tablets (Podczeck and Newton, 2003, Sonnergaard, 2013). These observations have raised concerns regarding the validity of BFI as a quantitative measure of tablet brittleness (Mazel and Tchoreloff, 2023, Meng et al., 2021, Sonnergaard, 2023, Sonnergaard, 2013). The BDI was subsequently developed to characterize tablet brittleness or ductility using the force–displacement curve obtained during tablet fracture testing (Sonnergaard, 2013). By considering the entire force–displacement response, BDI provides a more mechanistic description of fracture propensity, as brittle tablets typically exhibit a smaller area under the curve than tablets composed of more plastic materials (Sonnergaard, 2023, Sonnergaard, 2013). However, both BFI and BDI were designed primarily to describe intrinsic material behavior rather than tablet-level properties, as they do not explicitly account for the internal tablet structure, such as porosity, in their quantification of brittleness (Hiestand et al., 1977, Hiestand and Peot, 1974, Hiestand and Smith, 1984, Podczeck and Newton, 2003, Sonnergaard, 2013). In comparison, TBI quantifies the brittleness of individual tablets while capturing the effects of tablet porosity (ԑ) and tablet tensile strength (σ).

This distinction is critical because tablets prepared from the same material can exhibit very different porosities and tensile strengths under different compaction conditions (Gong and Sun, 2015, Paul and Sun, 2017). Using TBI, it has been shown that tablets become more brittle at higher porosity or lower tensile strength (Gong et al., 2015, Gong and Sun, 2015, Paul and Sun, 2017). Moreover, TBI has been correlated with tablet friability (Gong et al., 2015, Osei-Yeboah and Sun, 2015). Accordingly, TBI is employed in the present work to quantify brittleness.

It is well recognized that material plasticity plays an important role in the compaction behavior of powders (Elsergany et al., 2020a, Elsergany et al., 2020b, Heckel, 1961a, Heckel, 1961b, Tye et al., 2005). When all other factors, such as particle size and shape, are held constant, a more plastic powder undergoes more extensive plastic deformation under identical compaction conditions, leading to a larger bonding area and, hence, stronger tablets (Shi and Sun, 2024, Wang et al., 2024, 2023). Among the parameters used to quantify powder plasticity (Elsergany et al., 2023, Sun, 2005, Sun, 2004, Vreeman and Sun, 2022a, Vreeman and Sun, 2021), the in-die mean yield pressure, P_y,i, is considered the most material-sparing metric (Vreeman and Sun, 2021). Typically, materials with higher P_y,i values tend to fragment more easily under applied stress (Elsergany et al., 2023, Elsergany et al., 2020a, Elsergany et al., 2020b, Vreeman and Sun, 2022a, Vreeman and Sun, 2021). Given these empirical observations, it is reasonable to hypothesize that more plastic powders (i.e., those with lower P_y,i) would exhibit lower TBI at the same tablet porosity. However, to date, such a correlation has not been experimentally demonstrated. Therefore, the primary objective of this study was to investigate the potential relationship between P_y,i and TBI using a diverse set of 19 powders (7 pure powders and 12 binary mixtures). Additionally, we re-examined the previously reported dependence of TBI on tablet porosity and tensile strength using these new materials set.

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Materials

All materials were used as supplied by respective suppliers. Microcrystalline cellulose (MCC; Avicel PH101, PH102 and PH200) were from FMC Biopolymer (Philadelphia, Pennsylvania, USA), where the median particle sizes (d₅₀) of MCC PH101, MCC PH102 and MCC PH200 were 59 μm, 108 μm and 197 μm, respectively. Lactose monohydrate (LMH, d₅₀ = 125 μm; Fastflo® 316) was from Foremost Farms (Clayton, Wisconsin, USA). Crospovidone (PVPP, d₅₀ = 20 μm; Kollidon® CL-SF) and polyvinylpyrrolidone K25 (PVP K25).

Ramy N. Elsergany, Changquan Calvin Sun, Relationships among material plasticity, tablet brittleness, and tensile strength, International Journal of Pharmaceutics, Volume 692, 2026, 126658, ISSN 0378-5173, https://doi.org/10.1016/j.ijpharm.2026.126658.