Urea-formaldehyde Microcapsules Research Articles

Durability of self-healing woven glass fabric/epoxy composites

In this work, the durability of the healing capability of self-healing woven glass fabric/epoxylaminates was investigated. The composites contained a two-component healing systemwith epoxy-loaded urea-formaldehyde microcapsules as the polymerizable binder andCuBr2(2-methylimidazole)4 (CuBr2(2-MeIm)4) as the latent hardener. It was found that the healing efficiency of the laminates firstlydecreased with storage time at room temperature, and then leveled off for over twomonths. By means of a systematic investigation and particularly verification testswith dynamic mechanical analysis (DMA), diffusion of epoxy monomer from themicrocapsules due to volumetric contraction of the composites during manufacturingwas found to be the probable cause. The diffusing sites on the microcapsuleswere eventually blocked because the penetrated resin was gradually cured by theremnant amine curing agent in the composites’ matrix, and eventually the healingability was no longer reduced after a longer storage time. The results shouldhelp to develop approaches for improving the service stability of the laminates.

Novel Glass Fiber-reinforced Cyanate Ester/Microcapsule Composites

Novel fiber-reinforced dicyanate of bisphenol A (BADCy)/poly (urea-formaldehyde) microcapsules filled with epoxy resins (MCEs) composites were prepared. The effects of MCEs with different content and diameter on the mechanical properties and the hot-wet resistance of the composites were discussed. The morphologies of composites were characterized using a scanning electron microscopy and optical microscopy. The dynamic mechanical property (DMA) of the composites was also evaluated. The results indicate that the appropriate addition of MCEs with various diameters can significantly improve the mechanical properties and the hot-wet resistance of fiber-reinforced BADCy composites. The fiber-reinforced BADCy/MCEs composites have lower glass transition temperature (Tg), compared with the fiber-reinforced BADCy composites without MCEs.

Self-healing woven glass fabric/epoxy composites with the healant consisting ofmicro-encapsulated epoxy and latent curing agent

This paper reports a study of self-healing woven glass fabric reinforced epoxy composites.The healing agent was a two-component one synthesized in the authors’ laboratory, whichconsisted of epoxy-loaded urea-formaldehyde microcapsules as the polymerizable binder andCuBr2(2-methylimidazole)4 (CuBr2(2-MeIm)4) as the latent hardener. Both the microcapsules and the matching catalyst werepre-embedded and pre-dissolved in the composites’ matrix, respectively. Whenthe microcapsules are split by propagating cracks, the uncured epoxy can bereleased into the damaged areas and then consolidated under the catalysis ofCuBr2(2-MeIm)4 that was homogeneously distributed in the composites’ matrix on a molecular scale. As aresult, the cracked faces can be bonded together. The influence of the content of theself-healing agent on the composites’ tensile properties, interlaminar fracturetoughness and healing efficiency was evaluated. It was found that a healing efficiencyover 70% relative to the fracture toughness of virgin composites was obtainedin the case of 30 wt% epoxy-loaded microcapsules and 2 wt% latent hardener.

Fatigue crack propagation in microcapsule-toughened epoxy

The addition of liquid-filled urea-formaldehyde (UF) microcapsules to an epoxy matrix leads to significant reduction in fatigue crack growth rate and corresponding increase in fatigue life. Mode-I fatigue crack propagation is measured using a tapered double-cantilever beam (TDCB) specimen for a range of microcapsule concentrations and sizes: 0, 5, 10, and 20% by weight and 50, 180, and 460 μm diameter. Cyclic crack growth in both the neat epoxy and epoxy filled with microcapsules obeys the Paris power law. Above a transition value of the applied stress intensity factor ΔK T, which corresponds to loading conditions where the size of the plastic zone approaches the size of the embedded microcapsules, the Paris law exponent decreases with increasing content of microcapsules, ranging from 9.7 for neat epoxy to approximately 4.5 for concentrations above 10 wt% microcapsules. Improved resistance to fatigue crack propagation, indicated by both the decreased crack growth rates and increased cyclic stress intensity for the onset of unstable fatigue-crack growth, is attributed to toughening mechanisms induced by the embedded microcapsules as well as crack shielding due to the release of fluid as the capsules are ruptured. In addition to increasing the inherent fatigue life of epoxy, embedded microcapsules filled with an appropriate healing agent provide a potential mechanism for self-healing of fatigue damage.

Self-healing epoxy composites – Preparation and effect of the healant consisting of microencapsulated epoxy and latent curing agent

To provide epoxy based composites with self-healing ability, two-component healing system consisting of urea–formaldehyde microcapsules containing epoxy (30–70 μm in diameter) and CuBr 2(2-MeIm) 4 (the complex of CuBr 2 and 2-methylimidazole) latent hardener was synthesized. When cracks were initiated or propagated in the composites, the neighbor microencapsulated epoxy healing agent would be damaged and released. As the latent hardener is soluble in epoxy, it can be well dispersed in epoxy composites during composites manufacturing, and hence activate the released epoxy wherever it is. As a result, repair of the cracked sites is completed through curing of the released epoxy. The present paper studied the preparation of epoxy microcapsules by amino resins, and the influencing factors as well. On the basis of this work, mechanical properties of the epoxy filled with the healing system were evaluated. It was found that incorporation of the two-component healing system nearly did not change the fracture toughness of the neat epoxy, as indicated by the single-edge notched bending test. In the case of 10 wt% microcapsules and 2 wt% latent hardener, the self-healing epoxy exhibited a 111% recovery of its original fracture toughness. Besides, the preliminary result of double-cantilever beam testing showed that the plain weave glass fabric laminates using the above self-healing epoxy as the matrix received a healing efficiency of 68%.

Self-healing coatings for steel

The efficacy of a “self-healing” corrosion protection coating system for use on steel enclosures for outdoor equipment has been investigated using urea formaldehyde microcapsules (50–150 μm in diameter) containing several types of film forming compounds (healants) and corrosion inhibitors mixed into commercially available coatings systems. Five different types of inhibitors/film formers were tested, and three different techniques for application of the coatings with microcapsules were evaluated. Laboratory tests showed that when the coating system was damaged by abrasion, the microcapsules released the film forming and corrosion inhibiting compounds. Steel substrates coated with these self-healing systems were scribed and laboratory tested according to ASTM D 5894. Undercutting at the scribe (ASTM D 1654) was reduced by using microcapsules containing self-healing compounds. Growth of coating damage at the scribe was arrested in self-healing coatings with all microcapsule formulations compared to control samples. The performance of some microcapsules evaluated in this study was found to be dependent on the method of application.

A Methodology for Damage Strength Evaluation of a Single Biomimetic Microcapsule

An ultra-precision instrument with concomitant micromanipulation techniques is designed and set up to measure the damage strength of a single biomimetic microcapsule. It can provide the capability of simultaneously measuring the applied force and resultant displacement of a single microcapsule, with maximum force range of 5mN, resolution of 0.1µN and ultimate traveling distance up to 12mm, resolution of 1nm, respectively. By armed high magnification side-view system, it can offer extra and withal valuable information for the supervened analyzing. The bursting force of urea-formaldehyde biomimetic microcapsules of diameter 65µm in glucose solution was measured by this technique. The microcapsule was burst when the deformation reached a value of 56.2% of its diameter and the corresponding resonant force is about 1700µN. The technique provides an effective means to characterize elastic properties of micro biomimetic capsules and compare mechanical strength of microcapsules made of different ingredients.

The relation between narrow-dispersed microcapsules and surfactants

Electric ink display is one of the prospect technologies in paper-like display. In this paper, electric ink microcapsules are prepared by means of an in situ polymerization and complex coacervation. In order to obtain the microcapsules in a uniform size distribution, this study focused on the inter-facial tension between tetrachloroethylene and water solution, the dispersion of the core droplets and the microencapsulation with different kind of surfactants. By measuring the inter-facial tensions between water and tetrachloroethylene, it is found that urea-formaldehyde (UF) pre-polymer presents certain surface activity due to the inter-facial tension descending from 43 mN m−1 to 35 mN m−1. Because the surface activity of pre-polymer is not valid, the water-soluble emulsifiers can occupy on the surface of the droplets and prevent the UF resin depositing there. The analysis of the size distribution shows that the UF microcapsules are multi-dispersed. Furthermore, the influence of anionic surfactant of SDS on the size distribution of the core droplets is also investigated. The average diameters of the core droplets prepared with 0.005 wt% and 0.012 wt% SDS are ∼50 µm and 28 µm, respectively. That reveals the existence of SDS not only decreases the droplet diameters, but also makes the size distribution centralized. Because the surface of the core droplets is charged due to the absorption of SDS anionic, the Gelatin and Gum Arabic (GA) coacervating layer is easy to form there. The size of the GA microcapsules prepared with 0.003 wt% SDS is ∼65 µm. Finally; the responsive behaviour of the microcapsules to electric field is also investigated.

Preparation of Microcapsules Containing Two-Phase Core Materials

Urea-formaldehyde (UF) microcapsules containing two-phase core materials in which phthalocyanine blue BGS (beta-CuPc) particles were homodispersed in tetrachloroethylene (TCE) were prepared by in situ polymerization. The effects of the various process parameters, including the type of surface modifier, the viscosity of UF prepolymer, the type of water-soluble surfactant, and the concentration of oil-soluble surfactant in the capsule core on the dispersity of beta-CuPc particles in TCE and the properties of the capsule wall and the adsorption of beta-CuPc particles on the internal surface of capsule wall were experimentally investigated. It was shown that using octadecylamine (ODA) to modify beta-CuPc particles resulted in a significant increase of the dispersing extent (DE) and the electrophoresis velocity of the particles in TCE (about 4 and 20 times more than that of unmodified). In addition, the optimal reaction conditions of the synthesis UF prepolymer were obtained by the orthogonal test. On the other hand, as the oil/water interfacial tension of emulsion was big enough, the microcapsule formed. The concentration of Span-80 in TCE was no less than 0.062 mM; the adsorption of beta-CuPc particles on internal surface of wall were restrained. Finally, the microcapsules in which beta-CuPc particles possess reversible response to dc electric field were obtained.

Microcapsule induced toughening in a self-healing polymer composite

Microencapsulated dicyclopentadiene (DCPD) healing agent and Grubbs' Ru catalyst are incorporated into an epoxy matrix to produce a polymer composite capable of self-healing. The fracture toughness and healing efficiency of this composite are measured using a tapered double-cantilever beam (TDCB) specimen. Both the virgin and healed fracture toughness depend strongly on the size and concentration of microcapsules added to the epoxy. Fracture of the neat epoxy is brittle, exhibiting a mirror fracture surface. Addition of DCPD-filled urea-formaldehyde (UF) microcapsules yields up to 127% increase in fracture toughness and induces a change in the fracture plane morphology to hackle markings. The fracture toughness of epoxy with embedded microcapsules is much greater than epoxy samples with similar concentrations of silica microspheres or solid UF polymer particles. The increased toughening associated with fluid-filled microcapsules is attributed to increased hackle markings as well as subsurface microcracking not observed for solid particle fillers. Overall the embedded microcapsules provide two independent effects: the increase in virgin fracture toughness from general toughening and the ability to self-heal the virgin fracture event.

Controlled degradation of cured natural rubber by encapsulated benzophenone as a photosensitizer

The photodegradation of raw natural rubber and natural rubber compound film were studied using an artificial solar energy simulator. The properties of degraded rubber sheets containing benzophenone (BP) were determined by solution viscosity, 1H-NMR, and FTIR analyses. In the case of rubber compounds containing BP, the changes of tensile strength and crosslinking density were determined. It was found that BP could amply accelerate the photodegradation of rubber. To control the release rate of BP, it was necessary to encapsulate BP with urea–formaldehyde as a matrix. The encapsulated BP or capsule was formed by an interfacial polycondensation reaction between formaldehyde and urea. The kinetic of release rate of BP from urea–formaldehyde capsule was markedly observed within 15 days of release time; after that the rate of BP released from urea–formaldehyde microcapsule was very slow. At the same concentration of BP, the degradation rate of rubber compound by adding BP directly was faster than that of the rubber containing encapsulated BP. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 297–305, 2003

In situ poly(urea-formaldehyde) microencapsulation of dicyclopentadiene.

Microencapsulated healing agents that possess adequate strength, long shelf-life and excellent bonding to the host material are required for self-healing materials. Urea-formaldehyde microcapsules containing dicyclopentadiene were prepared by in situ polymerization in an oil-in-water emulsion that meet these requirements for self-healing epoxy. Microcapsules of 10-1000 microm in diameter were produced by appropriate selection of agitation rate in the range of 200-2000 rpm. A linear relation exists between log(mean diameter) and log(agitation rate). Surface morphology and shell wall thickness were investigated by optical and electron microscopy. Microcapsules are composed of a smooth 160-220 nm inner membrane and a rough, porous outer surface of agglomerated urea-formaldehyde nanoparticles. Surface morphology is influenced by pH of the reacting emulsion and interfacial surface area at the core-water interface. High yields (80-90%) of a free flowing powder of spherical microcapsules were produced with a fill content of 83-92 wt% as determined by CHN analysis.

In situ poly(urea-formaldehyde) microencapsulation of dicyclopentadiene

Microencapsulated healing agents that possess adequate strength, long shelf-life and excellent bonding to the host material are required for self-healing materials. Urea-formaldehyde microcapsules containing dicyclopentadiene were prepared by in situ polymerization in an oil-in-water emulsion that meet these requirements for self-healing epoxy. Microcapsules of 10–1000 μm in diameter were produced by appropriate selection of agitation rate in the range of 200–2000 rpm. A linear relation exists between log(mean diameter) and log(agitation rate). Surface morphology and shell wall thickness were investigated by optical and electron microscopy. Microcapsules are composed of a smooth 160–220 nm inner membrane and a rough, porous outer surface of agglomerated urea-formaldehyde nanoparticles. Surface morphology is influenced by pH of the reacting emulsion and interfacial surface area at the core-water interface. High yields (80–90%) of a free flowing powder of spherical microcapsules were produced with a fill content of 83–92 wt% as determined by CHN analysis.

Mechanical strength of microcapsules made of different wall materials.

The mechanical strength of microcapsules made of three different wall materials, including melamine-formaldehyde resin, urea-formaldehyde resin and gelatin-gum arabic coacervate, were measured by a micromanipulation technique. Single microcapsules were compressed to large deformations or rupture and the force being imposed on them were measured simultaneously. Melamine-formaldehyde and urea-formaldehyde microcapsules showed clear bursting under compression, and their bursting force, deformation at bursting and deformation at a pesudo yield point were determined. Gelatin microcapsules did not show clear bursting under compression, and their mechanical strength was characterized by the force required to cause their deformation to 50%. The mechanical strengths of these three types of microcapsules are compared in this paper.

Preparation and Characterization of Microcapsules Containing Lemon Oil

Urea–formaldehyde microcapsules were prepared by in situ interfacial polymerization to lemon oil as the core material using four kinds of emulsifier, gelatin, span 80, polyvinyl alcohol, and sodium dodecyl sulfate. Urea–formaldehyde types were investigated for their effects on thermal properties, mean particle size, and size distributions. Thermal properties were studied using a dynamic DSC and TGA analysis. The morphology and particle distributions of microcapsules were determined using an image analyzer. As experimental results, the diameters and distribution ranges of the microcapsules were decreased with increasing stirring rate and stirring time. In the presence of up to 5 g of emulsifier in 100 ml of deionized water, the mean diameter decreased, and then it increased as the emulsifier increased, resulting from increasing viscosity. The mean size of the microcapsules was related to the viscosity and reunification of core materials. That is, the mean size increased up to 22.8 cP due to increased the viscosity. But, at 30 cP, the mean size of the microcapsules decreased, as a consequence of the interruption of the reunion of core materials by the high viscosity of the core materials.