Fabrication and interfacial morphologies of methanol–melamine–formaldehyde (MMF) shell microPCMs/epoxy composites

Microcapsules containing phase change materials (microPCMs) have many potential applications because of their thermoregulation or thermosaving abilities. At the same time, it is still essential to understand the interface stability of microPCMs/polymer composites during a thermal transmission. The aim of this work was to fabricate novel microPCMs containing dodecanol by an in situ polymerization using methanol-modified melamine–formaldehyde (MMF) prepolymer as shell material and investigate the interface morphologies of microPCMs/epoxy composites treated by a simulant thermal process with a ten times repeated temperature variation. A series of microPCMs were fabricated by 1,000–3,000 r·min⁻¹ emulsion speed with the PCM contents of 40–70%. The average diameter, melting temperature, and encapsulation efficiency of microPCMs were 1–16 μm, 19.5 °C, and 97.4%, respectively. Tests results indicated that the properties of the microPCMs were greatly affected by core/shell ratios and emulsification stirring rates of preparation conditions. With the increasing of stirring rates, the average diameters of microPCMs were sharply decreased. The encapsulation efficiency (E _e) values of microPCMs increased with the increasing of stirring rates. The contents of PCM in microcapsules (C _t) and the average diameter of microPCMs both affected the interface morphologies of microPCMs/epoxy composites after the repeated thermal treatments. Microcracks and gaps occurred after a thermal treatment in the interface of microPCMs and epoxy matrix obviously. The internal stress generated by the expansion or shrinking of the microPCMs was the main factor leading to the interface morphology changes and damaged of composites.     

Thermal conductivity of microPCMs-filled epoxy matrix composites

Microencapsulated phase change materials (microPCMs) have been widely applied in solid matrix as thermal-storage or temperature-controlling functional composites. The thermal conductivity of these microPCMs/matrix composites is an important property need to be considered. In this study, a series of microPCMs have been fabricated using the in situ polymerization with various core/shell ratio and average diameter; the thermal conductivity of microPCMs/epoxy composites were investigated in details. The results show that the microPCMs have smooth surface and regular global shape with compact methanol–melamine–formaldehyde shell. The shell thickness does not greatly influence the phase change behaviors of PCM. Moreover, smaller microPCMs embedded in epoxy can improve the thermal transmission ability of composites. The effect of thermal conductivity of composites can be improved with higher volume fraction (10–30%) of microPCMs; and smaller size microPCMs with the same content of PCM may also enhance the thermal transmission area in matrix. Modeling analysis of relative thermal conductivity indicates that mixing higher thermal conductivity additive in PCM or matrix is an appropriate method to improve the thermal conductivity of microPCMs/matrix composites.     

High compact melamine-formaldehyde microPCMs containing n-octadecane fabricated by a two-step coacervation method

Microcapsules containing n-octadecane were successfully fabricated by an in-situ polymerization process with melamine-formaldehyde (MF) prepolymer and a hydrolyzed copolymer of styrene and maleic anhydride (SMA) as shell materials. To achieve a long service time of microcapsules containing phase change materials (microPCMs), the compactness of shells was improved by adding the MF prepolymer twice. The mechanism of this method was a two-step coacervation (TSC) under the help of hydrolyzed SMA compared to a one-step coacervation (OSC). To understand the influence of both coacervations, properties of shells were investigated in terms of morphologies, density, thickness, and stability by means of scanning electron microscopy (SEM), transmission electron microscopy, and thermal gravimetric analysis (TGA). The data of shells thickness were achieved from the cross-section SEM images. It shows that the average thickness of shells from two kinds of process are 0.1 μm. The density and stability in water of shells fabricated by TSC are both higher than that of shells by OSC. TGA curves show the expected microPCMs of TSC losing weight from 200 to 400 °C. The release curves, relationship between time and logarithmic residual weight of core, show there are two decrease-linear steps after curve regression. It can be concluded from all these results that the TSC method may be a promising method leading to a compact shell structure for various application.     

Physicochemical properties and mechanical characters of methanol-modified melamine-formaldehyde (MMF) shell microPCMs containing paraffin

Microcapsules containing phase change materials (microPCMs) with melamine-formaldehyde (MF) shells have been applied in many thermo-regulation or thermo-saving fields. However, it is still essential to decrease the residual formaldehyde and enhance the mechanical properties of MF shells. The objective of this work was to fabricate a series of microPCMs containing paraffin by an in situ polymerization method using methanol-modified melamine-formaldehyde (MMF) prepolymer as shell material and investigate the physicochemical properties and mechanical characters of these microPCMs. FT-IR analysis indicates that the methanol-modified method can reduce the free formaldehyde in shell material through increasing the cross-linking structure. Optical microphotographs and SEM morphologies show that the microPCMs have regular globe shape with smooth surface. With the increasing of emulsion stirring rates from 1,000 to 5,000 rpm, the average diameters decreased sharply from 27 to 2.5 μm. The phase change temperature (T _m) of microPCMs samples with the core/shell ratios of 3/1, 2/1, 1/1, and 2/1 are 22.6, 23.0, 23.4, and 23.9 °C, which are nearly equaled to the T _m of pure paraffin (22.5 °C). Mechanical properties test data show that the MMF shells have larger yield point value than that of MF shell for microPCMs with the same core/shell ratio, which means that the methanol-modified method shell can greatly enhanced the resistance of deformation for MF shells. Moreover, MMF shells can resist the interface extrusion force in epoxy resin owing to their higher yield point of enhanced MMF shell.     

Thermal stability of microencapsulated epoxy resins with poly(urea-formaldehyde)

A series of microcapsules filled with epoxy resins with poly(urea-formaldehyde) (PUF) shell were synthesized by in situ polymerization, and they were heat-treated for 2 h at 100 °C, 120 °C, 140 °C, 160 °C, 180 °C and 200 °C. The effects of surface morphology, wall shell thickness and diameter on the thermal stability of microcapsules were investigated. The chemical structure and surface morphology of microcapsules were investigated using Fourier-transform infrared spectroscope (FTIR) and scanning electron microscope (SEM), respectively. The thermal properties of microcapsules were investigated by thermogravimetric analysis (TGA and DTA) and by differential scanning calorimetry (DSC). The thermal damage mechanisms of microcapsules at lower temperature (<251 °C) are the diffusion of the core material out of the wall shell or the breakage of the wall shell owing to the mismatch of the thermal expansion of core and shell materials of microcapsules. The thermal damage mechanisms of microcapsules at higher temperature (>251 °C) are the decomposition of shell material and core materials. Increasing the wall shell thickness and surface compactness can enhance significantly the weight loss temperatures (T_d) of microcapsules. The microcapsules with mean wall shell thickness of 30 ± 5 μm and smoother surface exhibit higher thermal stability and can maintain quite intact up to approximately 180 °C.     

Synthesis and characterization of microencapsulated dicyclopentadiene with melamine–formaldehyde resins

Microcapsules containing healing agents have been used to develop the self-healing polymeric composites. These microcapsules must possess special properties such as appropriate strength and stability in surrounding medium. A new series of microcapsules containing dicyclopentadiene (DCPD) with melamine–formaldehyde (MF) resin as shell material were synthesized by in situ polymerization technology. These microcapsules may satisfy the requirements for self-healing polymeric composites. The chemical structure of microcapsule was identified by using Fourier transform infrared (FTIR) spectrometer. The morphology of microcapsule was observed by using optical microscope (OM) and scanning electron microscope. Size distribution and mean diameter of microcapsules were determined with OM. The thermal properties of microcapsules were investigated by using thermogravimetric analysis and differential scanning calorimetry. Additionally, the self-healing efficiency was evaluated. The results indicate that the poly(melamine–formaldehyde) (PMF) microcapsules containing DCPD have been synthesized successfully, and their mean diameters fall in the range of 65.2∼202.0 μm when the adjusting agitation rate varies from 150 to 500 rpm. Increasing the surfactant concentration can decrease the diameters of microcapsules. The prepared microcapsules are thermally stable up to 69 °C. The PMF microcapsules containing DCPD can be applied to polymeric composites to fabricate the self-healing composites.     

Structure and thermal stability of microencapsulated phase-change materials

A series of microcapsules containing n-octadecane with a urea-melamine-formaldehyde copolymer shell were synthesized by in-situ polymerization. The surface morphology, diameter, melting and crystallization properties, and thermal stability of the microcapsules were investigated by using FTIR, SEM, DSC, TGA and DTA. The diameters of the microcapsules are in the range of 0.2–5.6 m. The n-octadecane contents in the microcapsules are in the range of 65–78wt%. The mole ratio of urea-melamine has been found to have no effect on the melting temperature of the microcapsules. Two crystallization peaks on the DSC cooling curve have been observed. The thermal damage mechanisms are the liquefied n-octadecane leaking from the microcapsule and breakage of the shell due to the mismatch of thermal expansion of the core and shell materials at high temperatures. The thermal stability of materials can be enhanced up to 10 °C by the copolymerization of urea, melamine and formaldehyde in a mole ratio 0.2:0.8:3. The thermal stability of 160 °C heat-treated microcapsules containing 8.8% cyclohexane can be further enhanced up to approximately 37 °C.     

Optimization of parameters in preparation of PCM microcapsules based on melamine formaldehyde through dispersion polymerization

Microencapsulated phase change materials have attracted special attention due to their wide applications in saving and releasing energy. Here, microencapsulation of hexadecane (HD) in melamine formaldehyde shell was carried out through in situ dispersion polymerization in the aqueous media. Some important parameters such as stabilizer type and amount, surfactant amount, homogenization conditions as the critical affective factors on final particle size, morphology, and thermal resistance of the microcapsules were investigated extensively. The obtained microcapsules were concurrently analyzed by SEM, differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA) techniques. SEM images showed that the best stabilization was achieved by polyvinyl alcohol. Also, particle size, as an indication of surface area for heat transfer properties, showed a decrement by increasing stabilizer amount, surfactant amount, and homogenization speed. The amount of entrapped HD and efficiencies of microencapsulation were determined by DSC, and the reason for observing such changes were discussed in detail. Thermal stability of the microcapsules as an important property for their performance was investigated, too. The results illustrated that an improved thermal stability would be obtained by an efficient stabilization in the emulsification step. Also the highest thermal stability up to 388 °C was reached at homogenization speed of 6,000 rpm. Finally, the optimized conditions for desirable encapsulation were proposed in such systems.     

Preparation of microencapsulated ammonium polyphosphate with montmorillonite‐melamine formaldehyde resin and its flame retardancy in EVM

Microencapsulated ammonium polyphosphate (MMT‐MF‐APP) with a montmorillonite‐melamine formaldehyde resin coating layer was successfully prepared by in situ polymerization. The product was characterized by Fourier‐transform infrared, X‐ray photoelectron spectroscopy, and scanning electron microscopy. Water absorption analysis showed that the microencapsulation of APP with the MMT‐MF resin leads to a decrease in the particle's water solubility. The microcapsules also exhibited better mechanical properties and higher flame retardancy in the ethylene–vinyl acetate copolymer with high vinyl acetate content (EVM) rubber compared with the common ammonium polyphosphate. Moreover, thermogravimetric analysis results showed that the EVM composites with MMT‐MF‐APP and dipentaerythritol (DPER) as flame retardants possess higher thermal stability than those with common APP and DPER as flame retardants. Copyright © 2011 John Wiley & Sons, Ltd.     

Microencapsulation of styrene with melamine-formaldehyde resin

Melamine-formaldehyde (MF) resin-walled microcapsules containing styrene were prepared by in situ polymerization in an oil-in-water emulsion. In response to the characteristics of styrene (i.e., high volatility, non-polarity, low molecular weight, and low viscosity), the synthesis method was improved by optimizing the reaction conditions accordingly. It was found that utilization of macromolecular emulsifier of lower molecular weight, moderate dispersion rate, and higher feeding weight ratio of core/shell monomers is critical for the fast formation of capsules’ wall. The highest loading of styrene in the resultant microcapsules can be about 60%, and mean diameter of the capsules fell in the range of 20∼71 μm, which can be adjusted by changing processing parameters. It is believed that the present work provides a feasible approach to encapsulate monomers for manufacturing polyester based self-healing composites.     

Preparation of Porous Hollow SiO2 Spheres by a Modified St?ber Process Using MF Microspheres as Templates

Using the surface charged and acid dissolvable melamine formaldehyde (MF) microspheres as sacrificial hard templates, silica coated MF core?Cshell composite microspheres, denoted as MF@SiO₂, were synthesized via a surfactant-assisted sol?Cgel process by using tetraethyl orthosilicate (TEOS) as silica source. Hollow SiO₂ spheres with mesoporous shells were then obtained after selective removal of the MF cores and the pore directing surfactant by hydrochloric acid etching or calcinations in air. Interesting shrinkage phenomena were observed in both the hollow products derived from hydrochloric acid etching and calcinations. The influence of the ratio of MF sphere to TEOS and the removal method of the MF core on the size of the hollow spheres, the shell thickness and the shell surface roughness have been studied. The composition, the thermal stability, the morphology, the surface area and pore size distribution, the wall thickness and adsorption properties of the hollow spheres derived from hydrochloric acid etching and calcinations were also investigated and compared based on the FTIR, SEM, TEM, TGA, Nitrogen adsorption?Cdesorption and spectrophotometer techniques or measurements.     

Fabrication of hollow melamine-formaldehyde microcapsules from microbubble templates

A fabrication method for hollow melamine-formaldehyde microcapsules from microbubble templates is presented. This method is based on the direct encapsulation of microbubbles, and thus does not require a liquid- or solid-core decomposition process. This study determined the conditions for controlling the surface morphology, shell thickness, and diameter distribution of hollow microcapsules. Results showed that the surface morphology of these hollow microcapsules depended on the reaction time, glycine concentration (pH of aqueous continuous phase) and pre-polymer concentration. The capsule shell thickness could be controlled by adjusting the concentration of aniline that had adsorbed on the microbubble surface and reacted with pre-polymer. The capsule diameter depended on the dissolution rate of gases, and the diameter of the hollow microcapsules fabricated from air microbubble templates ranged from 5 to 200 microm.     

Preparation,characterization, and thermal properties of microPCMs containing <Emphasis Type="Italic">n</Emphasis>-dodecanol by using different types of styrene-maleic anhydride as emulsifier

Microcapsules containing polar phase change material (PCM) n-dodecanol were synthesized by in situ polymerization using melamine-formaldehyde resin as shell and styrene-maleic anhydride copolymer (SMA) as emulsifier. The effects of polarity of PCM and types and amounts of SMA emulsifier on the properties of microencapsulated PCMs (microPCMs) were studied and characterized by using Fourier transform infrared spectroscopy, scanning electron microscopy, differential scanning calorimetry, thermogravimetric analysis, and laser particle diameter analyzer. The results show that thermal properties of microPCMs are affected greatly by the types and amounts of SMA. Anionic SMA emulsifier is suitable for the encapsulation of n-dodecanol. The polarity of PCM leads to the higher adding amount of SMA emulsifier. When mass ratio of emulsifier to n-dodecanol is 4.8%, the phase change latent heat and encapsulation efficiency reach to the maximum value of 187.5 J/g and 93.1%, respectively. Irregular spherical microPCMs with mean diameter of 30.6 μm and phase change temperature of 21.5 °C are obtained and can be used for energy storage.     

Synthesis of functional microcapsules containing suspensions responsive to electric fields

A sort of functional microcapsules, which contain a suspension responsive to electric fields, is prepared by in situ polymerization of urea and formaldehyde. The suspension is made up of pigment phthalocyanine green (PPG) and tetrachloroethylene. In order to solve the particles' separation from the suspension during the microencapsulation and to obtain microcapsules applying to electronic ink display, the dispersibility of the particles, the contact angles between the particles and the tetrachloroethylene, and the influences of different emulsifiers on the microencapsulation are investigated. It is found that the dispersion extent and lipophilicity of the PPG particles are improved due to their surface modification with octadecylamine. The contact angles between the modified PPG particles and the tetrachloroethylene increase, and the PPG particles modified with 2 wt% octadecylamine have the best affinity for tetrachloroethylene. The interfacial tension between C(2)Cl(4) and H(2)O with urea-formaldehyde prepolymer descends from 43 to 35 mN/m, which indicates that the polymer has certain surface activity. However, water-soluble emulsifiers have an important influence during the microencapsulation because they can absorb on the surfaces of internal phase and prevent the resin of urea-formaldehyde from depositing there. From the SEM images of shell surface and cross section, the microcapsules have relatively smooth surfaces and the average thickness is about 4.5 mum. When the microcapsules are prepared with agitation rates of 1000 and 600 rpm, the mean diameters of the obtained microcapsules are 11 and 155 mum, respectively. The particles in the capsules move toward positive electrode with a responsive time of several hundred milliseconds while providing an electric field.     

Simultaneous determination of harpagoside and cinnamic acid in rat plasma by high-performance liquid chromatography: application to a pharmacokinetic study

Radix Scrophulariae (Xuanshen) is one of the famous Chinese herbal medicines widely used to treat rheumatism, tussis, pharyngalgia, arthritis, constipation, and conjunctival congestion. Harpagoside and cinnamic acid are the main bioactive components of Xuanshen. The purpose of this study was to develop an HPLC–UV method for simultaneous determination of harpagoside and cinnamic acid in rat plasma and investigate pharmacokinetic parameters of harpagoside and cinnamic acid after oral administration of Xuanshen extract (760 mg kg⁻¹). After addition of syringin as internal standard, the analytes were isolated from plasma by liquid–liquid extraction. Separation was achieved on a Kromasil C₁₈ column, and detection was by UV absorption at 272 nm. The described assay was validated in terms of linearity, accuracy, precision, recovery, and limit of quantification according to the FDA validation guidelines. Calibration curves for both analytes were linear with the coefficient of variation (r) for both was greater than 0.999. Accuracy for harpagoside and cinnamic acid ranged from 100.7–103.5% and 96.9–102.9%, respectively, and precision for both analytes were less than 8.5%. The main pharmacokinetic parameters found for harpagoside and cinnamic acid after oral infusion of Xuanshen extract were as follows: C _max 1488.7 ± 205.9 and 556.8 ± 94.2 ng mL⁻¹, T _max 2.09 ± 0.31 and (1.48 ± 0.14 h, AUC_0–24 10336.4 ± 1426.8 and 3653.1 ± 456.4 ng h mL⁻¹, 11276.8 ± 1321.4 and 3704.5 ± 398.8 ng h mL⁻¹, and t _1/2 4.9 ± 1.3 and 2.5 ± 0.9 h, respectively. These results indicated that the proposed method is simple, selective, and feasible for pharmacokinetic study of Radix Scrophulariae extract in rats. Figure Radix Scrophulariae     

硬脂酸丁酯微胶囊的制备与表征

采用原位聚合法用脲醛树脂包覆硬脂酸丁酯,制得相变储热微胶囊.利用激光粒径分布仪、扫描电镜、差示扫描量热仪(DSC)和傅立叶转换红外光谱仪分别研究了微胶囊的粒径分布、表面形态、热性能和壳结构.结果表明,所得微胶囊粒径分布均匀,表面光洁,具有良好的韧性和致密性.不同的制备工艺对微胶囊粒径分布有一定的影响,其中在28 000 r/m in下乳化5 m in时,所得微胶囊的粒径分布集中在1~4μm.DSC测定结果显示硬脂酸丁酯微胶囊的最大相变焓为68 J/g.     

Preparation of PCM microcapsules by complex coacervation of silk fibroin and chitosan

Phase change material microcapsules were prepared by complex coacervation of silk fibroin (SF) and chitosan (CHI). n-Eicosane was used as the core material. The effects of SF/CHI ratio, and percentage of cross-linking agent and n-Eicosane content on the properties of microcapsules were studied. The size distribution and the surface morphology of microcapsules were characterized by optical and scanning electron microscopy. The encapsulation of core material was determined by energy dispersive spectrometer analysis. The results indicated that SF/CHI microcapsules were prepared successfully. Microcapsules had smooth outer surface when the ratio of SF to CHI was close to 5. On the other hand, at high SF/CHI ratios (≥14), microcapsules showed a two-layer structure, an inner compact layer, and an outer, more porous, sponge-like layer. The highest microencapsulation efficiency was obtained at a SF/CHI ratio of 20 in the presence of 0.9% cross-linking agent and of 1.5% n-Eicosane content.     

Effect of preparative conditions on the surface characteristics of mixed zirconium and titanium oxides

The surface characteristics of mixed zirconium and titanium oxides prepared from different starting materials are investigated. One mode of preparation entailed the use of zirconium sulfate and titanium oxysulfate as starting materials and ammonium hydroxide as precipitating agent. The produced oxides were washed to different extents to obtain samples with different sulfate content. A second preparative mode used zirconium oxychloride and titanous chloride as starting materials also with ammonium hydroxide as precipitating agent. The oxidation of the titanous to the titanic form for these oxides was carried out by means of oxygen gas. Resulting samples were heat treated at 400 °C and 600 °C, and textural characteristics determined from the adsorption of N₂ at 77 K, complemented by infrared and thermal studies. The samples precipitated from the oxychloride and chloride salts of zirconium and titanium, as well as those precipitated from the sulfate and oxysulfate salts and washed free of the sulfate ions displayed quite similar textural characteristics. The unheated samples and those heat-treated at 400 °C were mesoporous, with some microporosity, and relatively large surface areas in the order of 200–300 m²/g. Heat treatment to 600 °C led to a relative decrease in surface area, in the order of 100 m²/g, and to the disappearance of microporosity. The mixed zirconium and titanium oxides with a sulfate content of ≈17% displayed significantly lower surface areas, smaller than 10 m²/g, with a prevalence of micro and mesoporosity. Infrared and thermal studies indicated the presence of differently bounded sulfato groups, which seem to have a blocking effect on the pores, resulting in the observed smaller surface areas.     

Synthesis of spherical microcapsules by photopolymerization in aerosols

 The preparation of polymer microcapsules of well defined size in the range of 10–50 μm with different shell thickness to core diameter ratios is described. An aerosol of monodisperse droplets of a homogeneous ternary liquid system which contained a hydrophobic component and a hydrophilic component dissolved in a high-volatile mutual solvent, was produced by dispersing with a vibrating-orifice aerosol generator. After the evaporation of the solvent in a nitrogen atmosphere the particles demix and form a two-phase droplet of core-shell type. These droplets were illuminated with UV light and polymerized to highly monodisperse microcapsules with a solid polymer shell and a liquid core. The properties of the resulting particles (size, size distribution, shell thickness, shape and surface characteristics) were investigated by scanning electron microscopy, Raman spectroscopy on single optically levitated particles, and confocal Raman micro spectroscopy. The microcapsules were highly monodisperse and have spherical shape. Received: 24 July 1996 Accepted: 29 August 1996     

Electrochemical etching of silicon carbide

Both n- and p-type SiC of different doping levels were electrochemically etched by HF. The etch rate (up to 1.5 μm/min) and the surface morphology of p-type 6H-SiC were sensitive to the applied voltage and the HF concentration. The electrochemical valence of 6.3 ± 0.5 elementary charge per SiC molecule was determined. At p-n junctions (p-type layer on a n-type 6H-SiC substrate) a selective etching of the p-type epilayer could be achieved. For a planar 6H-4H polytype junction (n-type, both polytypes with equal doping concentrations) the 4H region was selectively etched under UV illumination. Thus polytype junctions could be marked by electrochemical etching. With HCl instead of HF no etching of SiC occurs, but a SiO₂ layer (thickness up to 8 μm) is formed by anodic oxidation. Received: 29 October 1998 / Accepted: 27 January 1999