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Appleton, WI, United States

Mercade-Prieto R.,University of Birmingham | Allen R.,University of Birmingham | Zhang Z.,University of Birmingham | York D.,Procter and Gamble | And 2 more authors.
AIChE Journal | Year: 2012

Characterization of the failure behavior of microcapsules is extremely important to control the release of their core actives by mechanical forces. The strain and stress of elastic-plastic uninflated core-shell microcapsules at failure (rupture or bursting) has been determined using finite element modeling (FEM) and micromanipulation compression experiments. The ductile failure of polymeric microcapsules at high deformations is considered to occur when the maximum strain in the shell exceeds a critical strain, resulting in their rupture. FEM has been used to determine the maximum strains present in the capsule wall at different deformations for three types of shell material: elastic, elastic-perfectly plastic and elastic-perfectly plastic with strain hardening at large strains. The results obtained were used to determine the failure strain and stress of melamine-formaldehyde microcapsules, with average population values of ~0.48 and ~350 MPa, respectively. Thus, the elastic-plastic stress-strain relationship has been determined for the core-shell microcapsules tested. © 2011 American Institute of Chemical Engineers (AIChE). Source


Mercade-Prieto R.,University of Birmingham | Allen R.,University of Birmingham | York D.,Procter and Gamble | Preece J.A.,University of Birmingham | And 2 more authors.
Experimental Mechanics | Year: 2012

The encapsulation of liquids within an external wall or shell is an important technology often utilized in the production of many commercial products. The mechanical characterization of such microcapsules is paramount in order to fully understand their performance in their target environment. Some microcapsules, with wall materials such as inorganic based compounds, rupture at small deformations, commonly near the elastic regime. The study herein presents a general methodology that enables calculation of the failure stresses leading to the elastic-like rupture of microcapsules under parallel compression testing. Two scenarios of failure, brittle and ductile, were considered. Analyses of the critical stresses present within the microcapsule wall during different degrees of fractional deformation were obtained using finite element modelling, resulting in similar values for both the brittle and ductile scenarios. The correlations presented were used to determine the failure stresses of tetraethoxyorthosilane-methyltrimethoxysilane (TEOS-MTMS) microcapsules with a model core oil, which are 11-14 ± 10 MPa. The data were further analyzed using Weibull distributions. © 2012 Society for Experimental Mechanics. Source


Mercade-Prieto R.,University of Birmingham | Allen R.,University of Birmingham | York D.,Proctor and Gamble Technical Center | Preece J.A.,University of Birmingham | And 2 more authors.
Chemical Engineering Science | Year: 2011

Finite elements modelling (FEM) is used to simulate the compression response of core-shell microcapsules with an elastic-perfectly plastic wall material. Three methods are presented to calculate the yield stress of the material from experimental compression force curves. Dry melamine-formaldehyde microcapsules compressed using a micromanipulation technique were used as experimental validation. The calculated yield stress with the different methods all agreed within sample variability, at ~130. MPa. © 2011 Elsevier Ltd. Source


Mercade-Prieto R.,University of Birmingham | Nguyen B.,University of Birmingham | Allen R.,University of Birmingham | York D.,Procter and Gamble | And 3 more authors.
Chemical Engineering Science | Year: 2011

Finite element modeling (FEM) has been used to simulate the compression of single elastic core-shell capsules between two parallel plates. FEM allows characterizing the compressions at deformations beyond the wall thickness, when both bending and stretching contribute to the force resisting compression. Due to the incorporation of bending effects, the force deformation profiles of capsules with the same elastic modulus E depend on the wall thickness to capsule radius ratio (h/r). A model is presented that enables the (i) calculation of h/r from the individual compression force profiles at fractional deformations lower than 0.1, thus applicable for brittle capsules and for elastic-plastic capsules and (ii) calculation of Eh by comparison with FEM data at that h/r. Thus, the model allows the determination of E from compression data alone, as the wall thickness is also determined. The compression of melamine-formaldehyde capsules with a hexyl salicylate core using a micromanipulation technique is given as an example of the application of the model. The estimated wall thickness value, found to be independent of the capsule size, is in excellent agreement with transmission electron microscopy measurements. © 2011 Elsevier Ltd. Source

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