Mechanical Properties Of Microcapsules Research Articles

Versatile Fabrication of Polymer Microcapsules with Controlled Shell Composition and Tunable Performance via Photopolymerization

In this work, a series of polymer microcapsules based on UV-curable prepolymers are prepared by combining an emulsion template and photopolymerization. The modulation of the shell structure is achieved by employing UV-curable prepolymers with different chemical structures (polyurethane acrylates, polyester acrylates, and epoxy acrylates) and functionalities (di-, tetra-, and hex-). The relationships between the shell structure and the microcapsule properties are investigated in detail. The results show that the properties of the microcapsules can be effectively regulated by adjusting the composition and cross-linking density of the shell. Epoxy acrylate-based microcapsules exhibit higher impermeability, solvent resistance, and barrier and mechanical properties than polyurethane acrylate and polyester acrylate-based microcapsules. Using UV-curable prepolymer with high functionality as a shell-forming material could effectively improve the impermeability, solvent resistance, and barrier and mechanical properties of microcapsules. In addition, the dispersion of microcapsules in the coating matrix tends to follow the "similar component, better compatibility" principle, i.e., a uniform dispersion of the microcapsule in the coating matrix is more easily achieved when the compositions of the microcapsule shell and coating are similar in structure. The convenient adjustment of the shell structure and the investigation of the "structure-property" relationship provide guidance for the further controlled design of microcapsules.

Experimental and multi‐scale finite element modeling for evaluating healing efficiency of electro‐sprayed microcapsule based glass fiber‐reinforced polymer composites

AbstractMicrocapsule based glass fiber‐reinforced polymer (GFRP) composites have attracted enormous attention due to enhancement of structures' longevity, reducing expenses, and simplicity of fabrication process. In this study, a micromechanical model of a woven E‐glass/epoxy composite containing microcapsules was developed based on finite element analysis (FEA). Modified sequential adsorption algorithm was chosen to generate and disperse microcapsules in three types of representative volume elements (RVEs). Also, mechanical properties of microcapsules and composite were assigned based on nanoindentation tests and standard experimental tests, respectively. Eventually, multi‐scaling method was implemented to homogenize maximum tensile stress, and subsequent evaluation of tensile after impact (TAI) healing efficiency. Therefore, a healing efficiency of 71% was obtained based on three types of simulations encompassing low‐velocity impact (LVI) and quasi‐static tensile tests on the RVEs. In order to validate the results; first, an electrospraying set‐up was exploited to fabricate multicore microcapsules. The fabricated microcapsules contained mercaptan hardener and epoxy resin as the healing agents, and they were covered by alginate shell. Second, incorporated composite specimens with microcapsules were fabricated via the hand‐layup method. The average TAI healing efficiency of 67% was achieved by experimental tests. Furthermore, scanning electron microscope images of the fractured surfaces confirmed rupture of microcapsules in the LVI test.

Mechanical characterization of fish oil microcapsules by a micromanipulation technique

This paper aims to study the mechanical properties of fish oil microcapsules made of different food-grade formulations and processing conditions, which is crucial to maintaining their mechanical integrity in further compaction for developing the final dosage form of tablet. The combination of gelatin and gum Arabic or maltodextrin were use as wall materials. Spray drying, complex coacervation, and double encapsulation (coacervation followed by spray coating) were used to prepare different fish oil microcapsules. Both fundamental physical properties and mechanical properties of microcapsules were investigated. The mechanical characterization results were obtained using a micromanipulation technique and revealed that the mean rupture forces of individual microcapsules ranged from 0.44 ± 0.11 to 1.73 ± 0.27 mN and tended to increase with increasing particle size. The Young's modulus of microcapsules was determined by fitting the experimental force versus displacement data with the Hertz model. The microcapsules prepared by coacervation of gelatin and gum Arabic followed by spray coating with a mixture of gelation and maltodextrin were the strongest and stiffest based on the calculated nominal rupture stress and Young's modulus respectively. These findings can provide guidance on choosing the most desirable formulation and processing conditions to produce strong microcapsules with desirable barrier properties for industrial applications.

Investigation on the mechanical properties of polyurea (PU)/melamine formaldehyde (MF) microcapsules prepared with different chain extenders

There is lack of understanding on controlling of mechanical properties of moisture-curing PU/MF microcapsules which limited its further application. PU/MF microcapsules containing a core of isophorone diisocyanate (IPDI) were prepared with different chain extenders, polyetheramine D400, H2O, triethylenetetramine and polyetheramine (PEA) D230 by following a two-step synthesis method in this study. Fourier transform infra-red (FTIR) spectroscopy, Malvern particle sizing, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). And micromanipulation technique was used to identify chemical bonds in the shell, size distributions, structure, thickness, and mechanical properties of microcapsules. The results show that PU/MF microcapsules were successfully prepared. Tr increased from 46.4 ± 13.9 N/m to 75.8 ± 23.3 N/m when extender changed from D400 to D230. And the Tr increased from 51.3 ± 14.1 to 94.8 ± 17.5 N/m when the swelling time increased from 1 to 3h. Morphologies of the shell were utilised to understand the mechanism of reactions in forming the shell materials.

Synthesis, Characterizations and Mechanical Properties of Microcapsules with Dual Shell of Polyurethane (PU)/Melamine Formaldehyde (MF): Effect of Different Chain Extenders

The mechanical properties and microstructure of moisture-curing polyurethane (PU)/melamine formaldehyde (MF) microcapsules are not clear yet because of the complicated oil/water formulation process. PU/MF microcapsules with isophorone diisocyanate as core were formulated with butanediol (BDO), polyethylene glycol (PEG) 400, PEG 1000, and PEG 2000 using a two-step synthesis method in this work. The resulted microcapsules were characterized using various techniques. Furthermore, their mechanical properties were measured using a micromanipulation technique, showing that PU/MF microcapsules were successfully synthesized with the different alcohol chain extenders and the main groups have been revealed by their FTIR spectra. The morphologies of the microcapsules displayed the differences caused by the chain extenders. Size distributions and mechanical properties were measured to study the effects of the length of alcohol chain extenders. TEM imagines with sharp interfaces between PU and MF layers and data of SE...

PH-Induced Softening of Polyelectrolyte Microcapsules without Apparent Swelling.

Polyelectrolyte microcapsules represent a versatile platform to encapsulate and release active ingredients. Understanding the effect of environmental conditions on the mechanical properties of microcapsules is critically important for enabling their applications under various settings. In this report, we investigate the effect of solution pH on the mechanical properties of polyelectrolyte microcapsules made of two weak polyelectrolytes (poly(acrylic acid) and branched poly(ethylenimine)), formed via recently introduced nanoscale interfacial complexation in emulsion (NICE). Interestingly, the stiffness of the NICE microcapsule shell is reduced significantly (by 2 orders of magnitude) when the solution pH is raised from 2 to 6 even though there is little change in the size of microcapsules. These seemingly counterintuitive results suggest that the molecular structure of NICE microcapsules may be different from those of conventional polyelectrolyte microcapsules. The possibility of tuning the shell stiffness without inducing significant changes in the microcapsule size could offer unique advantages in practical applications.

Estimation of the mechanical properties of urea–formaldehyde microcapsules by compression tests and finite element analysis

ABSTRACTInformation on the mechanical properties of microcapsules is essential to understanding their performance during manufacturing, processing, and applications. The mechanical properties of urea–formaldehyde (UF) microcapsules filled with ethyl phenyl acetate (EPA) were determined by applying single‐microcapsule compression and a finite element model. Microcapsules were prepared by in situ polymerization, and the average wall thickness of microcapsules was 192 ± 12 nm as determined using scanning electron microscopy. The deformation behavior of the microcapsule was measured by a single‐microcapsule compression experiment between two parallel plates. The results show that both burst deformation and burst force were linearly related to microcapsule size. A model for an elastic shell filled with incompressible fluid was used to simulate compression of the microcapsule in ABAQUS. Dimensionless parameters were introduced to the model. The relationship between dimensionless force and dimensionless displacement depended on the ratio of wall thickness to diameter (ε). However, the relationship remained the same when ε was less than 1% and can be fitted well by the mathematical equation. An estimation of Young's modulus can be obtained from the compression data for the dimensionless deformation from 10% to 15%. The average Young's modulus of a UF/EPA microcapsule is estimated to be 2.12 ± 0.45 GPa. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43414.

Ultrasonic synthesis of stable oil filled microcapsules using thiolated chitosan and their characterization by AFM and numerical simulations.

An experimental protocol has been developed for synthesizing stable core-shell microcapsules using a biopolymer, chitosan, lacking cross-linkable thiol functional groups. In the first step, thiol moieties were introduced into the backbone of chitosan using dl-N-acetylhomocysteine thiolactone (AHT). In the second step, AHT-modified chitosan shelled microcapsules, encapsulating an oil core, were successfully prepared using high intensity 20 kHz ultrasound. The size of chitosan and AHT modified chitosan microcapsules was found to be in the range of 1-15 μm. The thickness of the microcapsule shell increased with an increase in thiol content. The mechanical properties of microcapsules were evaluated by subjecting the microcapsules to compressive forces by colloidal probe AFM. The stiffness and the Young's modulus of the shell of microcapsules were determined by analyzing the force versus indentation data using Reissner's theory for indentation of thin elastic shells. The stiffness of AHT modified chitosan microcapsules was found to be higher than unmodified chitosan microcapsules. The viability of microcapsules to be embedded into processed food, pharmaceutical and cosmetic products was tested via numerical simulations. The confined capsule in the micro-channel was subjected to linear shear and uniform flows. We used finite element numerical simulations to determine the deformation of microcapsules in flow as a function of shear rate and thickness of the shell. The deformation of capsule was found to be linear with an increase in the shear rate. The deformation decreased with an increase in the thickness of the shell. Based on the simulations, we predict that the microcapsules would survive processing conditions and shear rates used in industrial applications.

Destruction of polylactic acid microcapsules under ultrasound irradiation

Hollow microcapsules have been considered for potential applications as drug or gene carriers. This paper describes an investigation into the mechanical properties of microcapsules with a biocompatible polylactic acid (PLA) shell that can be destroyed using ultrasound irradiation. The microcapsules had a radius of 1 to 25μm and a shell thickness of 100nm to 3μm, and their response to ultrasound pulses with a center frequency of 700kHz to 2MHz was investigated. It was found that approximately 50% of capsules with a radius of 20μm were destroyed using pulses with a pressure amplitude of 50kPa and a frequency of 700kHz, which is close to the resonance frequency of the capsules.

An efficient method for the output of new self-repairing materials through a reactive isocyanate encapsulation

An efficient method for the preparation of stable microcapsules with an industrially relevant core material was investigated for future use in self-repairing coatings. Microcapsules filled with isophorone diisocyanate (IPDI) were synthesized with different polymeric shells: polyurethane PU, poly(urea–formaldehyde) PUF and bi-layer polyurethane/poly(urea–formaldehyde) PU/PUF. Changing the encapsulation process by adding more shell wall monomers and pre-polymers allows the modulation of physical and mechanical properties of microcapsules. The thickness of microcapsules shell walls can be tuned for coating thicknesses and chemical environments. The effect of diverse process parameters and ingredients on the morphology of the microcapsules was observed by scanning electron microscopy (SEM) and optical microscopy (OM). Different techniques for the characterization of the chemical structure and the core content were considered such as Fourier transform infrared spectroscopy (FT-IR) as well as the characterization of thermal properties by differential scanning calorimetry (DSC). High yields of free flowing powder of spherical microcapsules were produced. The application of a liquid phase IPDI in self-healing polymer composite is studied. Diisocyanates, being reactive with water, introduce the possibility of achieving a really autonomous self-healing system in an aqueous or moisture-sensitive environment.

Design and construction of microcapsules containing rejuvenator for asphalt

Microcapsules containing rejuvenator are a promising chemical powder product applied in asphalt concrete to prolong its service life. However, the size, thermal stability and mechanical properties of microcapsules are strictly demanded to suit the natural instincts and the actual application conditions of asphalt. The objective of this work was to fabricate and characterize the physicochemical properties of novel microcapsules containing rejuvenator by an in-situ polymerization method using methanol–melamine–formaldehyde (MMF) prepolymer as shell material. It was found that the average size of microcapsules could be successfully controlled in the range of 23.5 to 5.0 μm with increasing stirring rates from 1000 to 6000 r·min− 1. The average size was mainly determined by the core material dispersion rates in an oil-in-water emulsion. Shell thickness and shell density were increased by adding more MMF prepolymer in the fabrication process. Thermal stability results indicated that the decomposition temperatures of microcapsule samples (core/shell ratios of 2/1, 1/1, 1/2 and 1/3) were higher than the melting temperature of asphalt (180 °C). Lower prepolymer dropping rate is responsible for the better homogeneity of the sample which results in achieving a correct thermal decomposition as the investigation in this case shows an improvement of the stability. The hardness and Young's modulus results measured by nanoindentation indicated that the MMF-shell microcapsules had an elastic–plastic deformation. The size and shell thickness were two main influencing factors of micromechanical properties of the microcapsules containing rejuvenator.

Investigation of mechanical properties of soft hydrogel microcapsules in relation to protein delivery using a MEMS force sensor

This article reports the investigation of mechanical properties of alginate-chitosan microcapsules and the relation to protein delivery. For microscale compression testing, a system based on a microelectromechanical systems (MEMS) capacitive force sensor was developed. The bulk microfabricated capacitive force sensors are capable of resolving forces up to 110 microN with a resolution of 33.2 nN along two independent axes. The monolithic force sensors were directly applied to characterizing mechanical properties of soft hydrogel microparticles without assembling additional end-effectors. Protein-loaded alginate-chitosan microcapsules of approximately 20 mum in diameter were prepared by an emulsion-internal gelation-polyelectrolyte coating method. Young's modulus values of individual microcapsules with 1, 2, and 3% chitosan coating were determined through microscale compression testing in both distilled deionized (DDI) water and pH 7.4 phosphate-buffered saline (PBS). Protein release rates were also determined in DDI water and PBS. Finally, protein release rates were correlated with mechanical properties of the microcapsules.

Microcapsules with low content of formaldehyde: preparation and characterization

In the research reported here, an in situpolymerization process has been used to produce melamine formaldehyde microcapsules containing an oil-based industrial precursor. Here the microcapsules were produced with a low formaldehyde to melamine molar ratio (0.20–0.49) compared to previous literature reports (2.30–5.50). The properties of the microcapsules such as morphology, particle diameter and distribution, wall thickness, mechanical strength, and encapsulation efficiency were characterized, and it was found that the wall thickness and mechanical properties of microcapsules were modulated as a function of formaldehyde to melamine (F/M) molar ratio. The wall thickness of the microcapsules measured by transmission electron microscopy (TEM) increased from 80 ± 1.9 to 308 ± 1.7 nm (285 ± 2.4% increase) when the F/M molar ratio was increased from 0.20 to 0.49 (145% increase), and the nominal rupture stress of the microcapsules measured by a micromanipulation technique increased from 1.3 ± 0.1 to 4.2 ± 0.4 MPa (223 ± 11.5% increase). In contrast, when the F/M molar ratio increased from 0.49 to 2.30 (369%), the wall thickness of microcapsules only increased by 14 ± 0.8% and the nominal rupture stress of the microcapsules only increased by 66 ± 12.8%. Thus, it has been shown that significant reduction in the levels of formaldehyde content is possible from previous literature reports, whilst only marginally reducing the mechanical properties, and still maintaining the encapsulation efficiency of ∼75%.

Mechanical Properties of Microcapsules Used in a Self-Healing Polymer

The elastic modulus and failure behavior of poly(urea-formaldehyde) shelled microcapsules were determined through single-capsule compression tests. Capsules were tested both dry and immersed in a fluid isotonic with the encapsulent. The testing of capsules immersed in a fluid had little influence on mechanical behavior in the elastic regime. Elastic modulus of the capsule shell wall was extracted by comparison with a shell theory model for the compression of a fluid filled microcapsule. Average capsule shell wall modulus was 3.7 GPa, regardless of whether the capsule was tested immersed or dry. Microcapsule diameter was found to have a significant effect on failure strength, with smaller capsules sustaining higher loads before failure. Capsule size had no effect on the modulus value determined from comparison with theory.