Melamine-formaldehyde Microcapsules Research Articles

Preparation and Characterization of Tung Oil Loaded Melamine Formaldehyde Microcapsules

In this study microcapsules were prepared by in-situ polymerization route with melamine formaldehyde as a shell material and tung oil as core material. Melamine formaldehyde (MF), a thermosetting polymer, is one of the most widely used monomers in microencapsulation due to its superior mechanical strength and thermal stability. Tung oil contains unsaturated double bonds that can be oxidized to form a film in air. Tung oil is fast drying and biodegradable, besides it is low cost and does not pollute the environment. Most importantly, tung oil is a versatile substance in industry. Therefore, tung oil is a good choice as core material. The chemical structure of microcapsules were characterized by Fourier Transform Infrared (FTIR) spectroscopy. The surface morphology and particle size and distribution were evaluated by Scanning Electron Microscopy (SEM). The thermal behavior of microcapsules and tung oil were studied by thermogravimetric analysis (TGA). The results showed that the spherical microcapsules (particle size of mostly 4-5 μm) were produced with a filling content of 15.64 wt.%, and a yield of 49.78 wt.%. The microcapsules exhibit a good thermal stability

Preparation and performance assessment of high-strength polyurea microcapsules

Formulation of microcapsules with high mechanical strength holds great significance for their application in high-pressure scenarios. In this study, polyurea (PU) was employed as the shell material to encapsulate perfume oil through interfacial polymerization. Subsequently, a systematic assessment of mechanical strength was conducted. The results indicated that the protective colloid, triethylenediamine (TEDA) concentration, core/shell ratio (perfume oil to polymethylene polyphenyl isocyanate (PAPI)), and reaction temperature, significantly influenced the mechanical strength of PU microcapsules. Notably, employing a concentration of 1.5 wt% polyvinyl alcohol (PVA), 0.5 wt% TEDA, a core/shell ratio of 10:1, and a reaction temperature of 70 °C resulted in PU microcapsules boasting an average rupture force of 1.92 ± 0.22 mN and a Young's modulus of 0.0195 ± 0.0013 GPa. Impressively, when contrasted with melamine-formaldehyde microcapsules, those with PU shells exhibited markedly enhanced mechanical properties.

Developing of capric acid @ colemanite doped melamine formaldehyde microcapsules and composites as novel thermal energy storage materials

Developing of capric acid @ colemanite doped melamine formaldehyde microcapsules and composites as novel thermal energy storage materials

Pullulan Films with PCMs: Recyclable Bio-Based Films with Thermal Management Functionality

The use of phase-changing materials (PCMs) is a practical and powerful way of buffering thermal fluctuations and maintaining the isothermal nature of the storage process. In this study, melamine formaldehyde microcapsules with paraffin cores were used as PCMs; pullulan films with PCMs were prepared by the film-casting method; and the composite films prepared were analysed with regard to their chemical structure, thermal properties, thermal stability, and recyclability. Uniform films displaying thermal management functionality were prepared. The amount of 75 wt.% PCM were added to the pullulan film structure which enabled the preparation of a composite film that displayed 104.85 J g−1 of heat storage during heating and 103.58 J g−1 of heat release during cooling. Multiple heating and cooling cycles showed that the composite films maintained their thermal management functionality after multiple heating-cooling cycles. The PCMs could be recovered with a yield of approximately 95% by the application of a simple dissolution and filtration process. The morphology, chemical structure, and thermal properties of the PCMs were maintained after the recovery process. The bio-based composite films with thermal management functionality and recyclability are proposed as an innovative, practical, and effective system for thermoactive storage and packaging applications.

Construction of consumer-friendly essential oil microcapsules with viscous cores to provide extra long-lasting release

Essential oil microcapsules with viscous cores were formulated by encapsulating oil droplets with intermediate non-crosslinked polymer shells and crosslinked polymer outer shells, followed by dissolution of the intermediate shells with essential oil to form viscous cores. The study found that when the core/shell ratios of the intermediate shell = 3:2, the rotational speed = 500 rpm, the concentration of the protective colloid = 1.0 wt%, the concentration of the crosslinking agent = 20 wt% and the core/shell ratio of the outer shell = 10:1 by using three times' addition method, caprylic/capric triglyceride (GTCC) microcapsules with viscous cores were obtained. The release profile, mechanical strength and adsorption performance of GTCC microcapsules were evaluated in comparison with melamine-formaldehyde microcapsules. It was found that the viscous cores microcapsules had better heat resistance, extreme long-lasting release property, viscoelasticity and been more adhesive to cotton fabrics, which were more consumer-friendly compared with that of melamine-formaldehyde microcapsules. • Construction of essential oil (GTCC) microcapsule with the viscous core structure. • Polystyrene (PS) was used as wall material, which is safe and consumer-friendly. • MF (Melamine-formaldehyde)-GTCC and PS-GTCC microcapsules were compared. • The release, viscoelasticity and adsorption property of microcapsule was enhanced.

Comprehensive evaluation on the encapsulation performances of melamine-formaldehyde microcapsules affected by core oils

Core oils’ properties are significant for the microencapsulation process. 12 representative single componential core oils with different chemical structures, logP, solubility, interfacial tension (IFT) and viscosity were specially selected to fabricate melamine-formaldehyde (MF) microcapsules. Afterward, the effect of core oils’ properties on the performance of microcapsules was investigated. It was found that the morphology, the average particle sizes (7.83 µm∼27.06 µm), the encapsulation efficiencies (47.35 ± 3%∼92.86 ± 4%), zeta potentials (−52.37 ± 2.97 mV∼−65.03 ± 0.79 mV) and the rupture forces (0.28 ± 0.20mN∼0.82 ± 0.3mN) of microcapsules were influenced by core oils’ properties. Further comprehensive analysis indicated that logP≥ 1.66 and the solubility≥ 0.055 mL3/g were the premises for the successful encapsulation processes. Moreover, by increasing logP, IFT and viscosity, the size of microcapsules increased; as microcapsules size increased, the release rate decreased; the high IFT and the solubility increased the encapsulation efficiency of microcapsules; the mechanical property was influenced by logP. This work provided to use logP as the criterion to predict the encapsulation work, which hasn’t been studied before; and the correlations between microcapsules’ various performances and core oils’ properties were initially established. This work was meaningful since logP was used as a clear criterion to select core oils for encapsulation work and core oils’ properties were used to predict the application performances of microcapsules.

Synthesis of pH sensitive microcapsules containing ZAPP, SAPP, and 8-HQ, and evaluation of their anti-corrosion performance, and mechanical enhancement of epoxy coating

Self-healing coatings containing microcapsules have a remarkable ability to preserve metal surfaces for a long time, which is why these smart coatings have received a lot of attention. In this study, pH-sensitive poly melamine-formaldehyde microcapsules were synthesized by in-situ polymerization and loaded into the epoxy coating. 8-HQ, ZAPP, and SAPP were used as inhibitors in the core of microcapsules (named MF8HQ, MFZAPP, and MFSAPP, respectively). The microcapsules were synthesized at different stirring times and rates to obtain the optimum state (800 rpm and 2.5 h), then characterized by SEM, TEM, and FT-IR tests. Also, the mechanical properties of coatings with different weight percentages of loaded microcapsules were investigated using tensile measurement, DMTA, and Pull-off tests. The results revealed that the presence of 5 wt% of microcapsules in coatings is along with more appropriate Tg, modulus, and adhesion strength compared to more or fewer amounts. Also, EIS and salt spray tests were used to evaluate the anti-corrosion properties of the coating. The results suggested that epoxy coating containing MF8HQ showed the best anti-corrosion performance after 35 days of immersion in 3.5 wt% of NaCl solution (delamination index = 45.7 %), followed by MFZAPP (delamination index = 69.5 %) and MFSAPP performance (delamination index = 84.2 %) which were better than neat epoxy, respectively. Finally, the Quantum analysis was used to ensure the 8-HQ inhibitory performance and analyze the mechanism of inhibiting action at the interface.

Effect of Transparent, Purple, and Yellow Shellac Microcapsules on the Optical Properties and Self-Healing Performance of Waterborne Coatings

Three kinds of melamine-formaldehyde (MF) microcapsules, containing transparent shellac, purple shellac, and yellow shellac as core curing agents, were synthesized via in situ polymerization, and then were embedded into the water-based acrylic resin coatings according to the concentrations of 0, 3.0%, 6.0%, 9.0%, 12.0%, and 15.0%, respectively, to obtain waterborne films with different microcapsule contents. The color of different shellacs was relevant to the color parameters of the self-healing waterborne film. The content of microcapsules was negatively correlated with the chromatic aberration of the surface of waterborne films. When the content of microcapsules was 0–6.0%, the chromatic aberration of waterborne films was relatively low. The content of microcapsules and the color of the different shellacs would affect the light transmittance of waterborne films. Among all samples, the light transmittance of the waterborne film containing 3.0% transparent shellac microcapsules was the highest. The microcapsules with different colors of shellac in waterborne films had different self-repairing effects. When the content of microcapsules did not exceed 6.0%, the tensile repair rate of the waterborne film containing yellow shellac encapsulated microcapsules was the highest, at 47.19%. The scratch experiment illustrated that the scratch width of the waterborne coating with yellow shellac microcapsules decreased most significantly, and the width change rate was 73.0% after 5 days. The coating containing the 3.0% yellow shellac microcapsule has the best comprehensive performance on optical and self-healing properties. Exploring the influence of shellac resin’s color and the microcapsules’ content on the waterborne film provides technical references for the application of shellac in waterborne coatings and contribute to the further development of the preparation process of self-healing coatings.

ZnO‐Reinforced Thermal Regulating Microcapsules with Double‐Layer Shell via Atomic Layer Deposition

AbstractConventional plain organic or inorganic shell for microcapsules has their own drawbacks, the organic shell has defects in thermal conductivity and stability; while the compactness and coverage of inorganic shell are not satisfactory. Herein, a novel approach to synthesis of organic–inorganic composite double‐layer shell phase change microcapsule (DLSPCM) is proposed, which is achieved by forming inorganic outer shell on the surface of organic shell via atomic layer deposition (ALD). Taking the pristine microcapsule with paraffin core and melamine formaldehyde (MF) shell as an example, MF‐ZnO composite double‐layer shell (DLS) is formed in this paper. The melting enthalpy of DLSPCM obtained by 100 ALD cycles is 159.7 J g−1, which is only 12.1 J g−1 lower than that of paraffin @ MF microcapsules (control sample), while the thermal conductivity of DLSPCM surges by 77.8%, and the thermal energy storage ability and thermal regulation performance are basically maintained. After 900 thermal cycles, the heat storage capacity of DLSPCM obtained by 100 ALD cycles decreased by only 3%. Besides, the photocatalytic performance and antibacterial activity of ZnO‐reinforced DLSPCM are further confirmed. The successful design of DLSPCM via ALD is prospective for the encapsulation, modification, and reinforcement of phase change microcapsules.

Fragrance oil microcapsules with low content of formaldehyde: Preparation and characterization

To reduce the residual formaldehyde content in melamine-formaldehyde (MF) microcapsules, glutaraldehyde was introduced to replace a portion of formaldehyde to prepare melamine-glutaraldehyde-formaldehyde (MGF) fragrance oil microcapsules via in-situ polymerization. MGF-lily oil microcapsules were obtained when the concentration of aldehyde groups provided by glutaraldehyde in the total aldehyde groups Cga∕Ca≤ 33.3 mol%. When Cga∕Ca≤ 9.5 mol%, the addition of glutaraldehyde reduced the residual formaldehyde content (235.05 ± 13.14 ppm to 21.79 ± 6.82 ppm) effectively without sacrificing the mechanical properties of microcapsules; it also improved the elasticity of microcapsules, in which Young’s modulus of MF shell was approximately 1.5 times to that of MGF shell. The approach was extended to encapsulate 2-hydroxy-3-(octanoyloxy)propyl decanoate (ODO), hexyl salicylate and lavender oil. All three fragrance oils were encapsulated successfully with low content of formaldehyde; however, the mechanical properties varied due to the difference on the hydrophobicity and viscosity of fragrance oil. This work provided an efficient and universal formaldehyde reduction method to encapsulate hydrophobic fragrance oil, which can not only reduce formaldehyde content but also maintain the mechanical properties and improve the elasticity of microcapsules, which has a potential application in consumer-friendly additives.

Study on release and inhibition action of mixed ZAPP and 8-HQ corrosion inhibitors loaded in pH-sensitive microcapsules for Mg AZ31

In-situ polymerization was used to synthesize pH-sensitive poly melamine-formaldehyde (PPMF)11pH-sensitive poly melamine-formaldehyde microcapsules containing 8-HQ228-Hydroxyquinoline and ZAPP33Zinc aluminum polyphosphate inhibitors. The chemical structure of PPMF microcapsules was confirmed using FTIR44Fourier transform infra-red spectroscopy, the mean particle size (2 µm) was determined using DLS55Diffraction light scattering, and the rough shape of the microcapsules was investigated using FESEM66Field-emission scanning electron microscopy. UV–visible spectroscopy77Ultraviolet-visible, EDX88Energy-dispersive x-ray spectroscopy, and ICP99Inductively coupled plasma were used to investigate inhibitors' release. The corrosion inhibition in coatings containing mixed encapsulated 8-HQ and ZAPP was suggested by the EIS1010Electrochemical impedance spectroscopy in 3.5 wt% NaCl solution by forming a protective layer, resulting in a significant increase in charge transfer and diffusion resistance, confirming the synergistic effect of mixed inhibitors. According to EIS measurements, long-lasting coatings (RCT= 16.8 MOhm.cm2 after 10 weeks) were produced by adding PPMF microcapsules containing mixed corrosion inhibitors due mainly to smart inhibitor release.

Enhanced mechanical properties of epoxy composites embedded with MF/TiO2 hybrid shell microcapsules containing n-octadecane

Microencapsulated phase change materials (MPCMs) are often mixed with matrix materials to form phase change composites for energy storage. Typically, MPCMs are easily debonded from the matrix or ruptured, thereby weakening the mechanical properties of composites. This paper aims to simultaneously improve the rupture strength of microcapsules and the bonding strength between microcapsules and matrix to enhance the mechanical properties of composites. The titanium dioxide (TiO2) nanoparticles modified by a silane coupling agent (KH560) were doped into the melamine formaldehyde (MF) shell, forming n-octadecane@MF/TiO2 hybrid shell MPCMs (HS-MPCMs). The doping of modified TiO2 nanoparticles reduced supercooling and improved the thermal stability of microcapsules. Compared with MF microcapsules, the rupture strength of HS-MPCMs was increased by an average of 30.4%. The modified TiO2 nanoparticles also built covalent bonds between microcapsule shell and matrix, which led to better microcapsule/epoxy interface bonding. Thus, the HS-MPCMs/epoxy composites performed higher tensile strength than the unmodified composites. Specifically, the tensile strength of composites was improved by an average of 17.2% at the microcapsule content of 10 wt.% with the aid of the MF/TiO2 hybrid shell. The reinforced MPCMs/epoxy composites are expected to be used as anti-icing coatings in the aerospace field.

Microencapsulation of Paraffin Wax in Melamine-Formaldehyde for use in Thermal Management Study

Melamine formaldehyde microcapsules containing paraffin wax as phase change material (PCM) were synthesized. Free space was generated in-situ inside microcapsules. These were characterized by FTIR Spectrophotometer, Scanning electron microscope (SEM), Differential Scanning colorimeter (DSC), and optical microscope. It was confirmed that PCM was successfully encapsulated inside the shell material. Encapsulated PCM showed good phase change properties during heating and cooling. Enthalpy value of about 100 J/g was observed. The shape of the microcapsule, showing rough morphology is expected to help during processing and crystallization. It has been experimentally confirmed that no leakage occurs during the melting of PCM. Effectiveness in temperature control in a hot environment was also found to be satisfactory.

Cerium containing pH-responsive microcapsule for smart coating application: Characterization and corrosion study

In the following paper first, novel pH-sensitive melamine-formaldehyde microcapsules containing ethylene glycol with three concentrations of cerium nitrate were synthesized. Then, three different quantities of the microcapsules were introduced to the epoxy coatings and applied on the AA2024-T3 substrate to meet the increasing demands for self-healing coatings. The characteristics of microcapsules were studied by scanning electron microscope (SEM), transmission electron microscope (TEM), Fourier transform infrared (FTIR), and dynamic light scattering (DLS). The UV–Vis spectroscopy was accomplished to the appraise pH-triggered release of the microcapsules. The salt spray test along with electrochemical impedance spectroscopy (EIS) was conducted for studying the anti-corrosion and self-healing proficiency of microcapsule-based coatings. The UV–Vis spectroscopy outcomes showed that the synthesized microcapsules are responsive to alkaline pH and the release rate of microcapsules in the basic medium rises with increasing the pH values. Moreover, the EIS and salt spray results indicated that incorporation of 10 wt% of the microcapsules into the epoxy coating not only improved the anti-corrosion strength of the aluminum substrate compared to neat epoxy coating remarkably but also, provided active corrosion protection of the substrate through the self-healing action of cerium ions. Also, the attendance of cerium cations in the scratch surface was confirmed by field emission scanning electron microscopy (FE-SEM) and energy-dispersive X-ray (EDX) map.

Melamine-formaldehyde microcapsules encapsulating HEDP for sustained scale inhibition

Adding scale inhibitors is an efficient and economical solution to the troublesome scale deposition in various industrial productions. However, the direct use of liquid scale inhibitors often fails to provide long-term effective control and will cause a large amount of waste of scale inhibitors. Encapsulating the scale inhibitor into microcapsules is an advisable new approach to solving this problem. In this work, a water-soluble scale inhibitor (1-hydroxy ethylidene-1,1-diphosphonic acid, HEDP) was encapsulated into melamine formaldehyde microcapsules for the first time by in-situ polymerization using inverse emulsion (W/O emulsion) as a template. The influence of synthesis parameters (water-to-oil ratio, the amount of HEDP, stirring speed) on the morphology and particle size was investigated, and microcapsules with smooth surface, regular spherical shape and good dispersibility were obtained. The microcapsules possessed encapsulation efficiency exceeding 80% and good sustained-release properties (release time up to 63 h) ensuring sustainable scale inhibition. The scale inhibitor released from the microcapsules could inhibit the growth of calcium carbonate crystals and result in lattice distortion, which contributed to high scale inhibition efficiency (about 86%). The prepared microcapsules have broad application prospects in the field of industrial scale inhibition. • A new way to use scale inhibitors, encapsulation of scale inhibitors, was provided. • A water-soluble scale inhibitor was successfully encapsulated into microcapsules. • Melamine-formaldehyde shell was used to encapsulate aqueous solution. • The microcapsules possessed high encapsulation efficiency. • The microcapsules possessed high scale inhibition efficiency.

Self-healing epoxy coating synthesis by embedment of metal 2-methyl imidazole and acetylacetonate complexes with microcapsules

The restoration of mechanical properties is desired for creating the self-healing coatings with no corrosion capabilities. The encapsulation of epoxy resins is limited by various factors in urea and melamine formaldehyde microcapsules. An improved method was developed, where epoxy resin was encapsulated by individual wrapping of poly(melamine-formaldehyde) and poly(urea-formaldehyde) shell around emulsified epoxy droplets via oil-in-water emulsion polymerization method. The synthesized materials were characterized analytically. The curing of the epoxy was achieved by adding the [Ni/Co(2-MI)6].2NO3 as a latent hardener and iron acetylacetonate [Fe(acac)3] as a latent accelerator. Isothermal and non-isothermal differential scanning calorimetric analysis revealed lower curing temperature (Tonset = 116 °C) and lower activation energies (Ea ≈ 69–75 kJ/mol). The addition of microcapsules and complexes did not adversely alter the flexural strength and flexural modulus of the epoxy coatings. The adhesion strength of neat coating decreased from 6310.8 ± 31 to 4720.9 ± 60 kPa and percent healing increased from 50.83 to 67.45% in the presence of acetylacetonate complex at 10 wt% of microcapsules.

Formulation of Next-Generation Multicompartment Microcapsules by Reversible Electrostatic Attraction.

The combination of several active substances into one carrier is often limited due to solubility, stability and phase‐separation issues. These issues have been addressed by an innovative capsule design, in which nanocapsules are assembled on the microcapsule surface by electrostatic forces to form a pH‐responsive hierarchical capsule@capsule system. Here, melamine‐formaldehyde (MF) microcapsules with a negative surface charge were synthesized and coated with a novel MF‐polyethyleneimine (PEI) copolymer to achieve a positive charge of ζ=+28 mV. This novel coating procedure allows the electrostatic assembly of negatively charged poly‐l‐lactide (PLLA, ζ=−19 mV) and poly‐(lactide‐co‐glycolide) (PLGA, ζ=−56 mV) nanocapsules on the microcapsule surface. Assembly studies at pH 7 gave a partial surface coverage of PLLA nanocapsules and full surface coverage for PLGA nanocapsules. The pH‐responsive adsorption and desorption of nanocapsules was shown at pH 7 and pH 3.

Efficient encapsulation of water soluble inorganic and organic actives in melamine formaldehyde based microcapsules for control release into an aqueous environment

Melamine formaldehyde (MF) is a versatile and widely used chemical cross-linking agent in microcapsule shells for the encapsulation of oil phases in an aqueous continuum. However, it has not been possible to form MF microcapsules with water soluble actives, to achieve sustained/no release in aqueous environment. Herein, 5 types of MF based microcapsules were designed and synthesised with 4 polymers, including neutral and polar poly(acrylamide-acrylic acid), anionically charged biocompatible shellac and ion exchange resin polystyrene sulfonate as well as a superhydrophobic polymer precursor octadecyltrichlorosilane, via an in situ polymerisation method. Potassium chloride and allura red (dye, Mw = 496.42 g·mol−1) were used as models of inorganic and organic water soluble actives with low molar mass, respectively, to study their size distributions, encapsulation efficiencies, payloads, morphologies and release rates in aqueous environment. The release rates ranged from complete release within 15 min to no release up to 1 month in aqueous environment.

Melamine-formaldehyde microencapsulated n-Tetracosane phase change material for solar thermal energy storage in coating

This work aims to prepare potential solar thermal energy storage coating using melamine-formaldehyde (MF) microcapsules with an n-Tetracosane (n-Tetra) core as phase change material (PCM). The shell material was prepared by reacting melamine with formaldehyde using a two-step process. After centrifuging and drying, these microcapsules were incorporated (0–25 wt%) into an epoxy primer-polyurethane (EP-PU) topcoat system for thermal energy storage applications. The microcapsules were characterized using Fourier Transform Infrared Spectroscopy (FTIR), Differential Scanning Calorimetry (DSC), Polarized Optical Microscopy (POM) & Scanning Electron Microscopy (SEM), respectively. n-Tetra, incorporated in the microcapsule (MF-n-Tetra), has been found to melt at 53.07 °C with the latent heat of melting 134.74 J/g and crystallizes at 48.81 with latent heat crystallization 133.32 J/g. The phase change properties of the coated material were found to increase with the increase in the microcapsule loading. The thermal energy transfer rate was quantified in terms of the time taken by the coated panels to reach an equilibrium temperature of 60 °C. The microcapsule incorporated coatings were also subjected to salt spray analysis to understand any potential effects of phase change material (PCM) loading on the corrosion resistance aspect of the primer-topcoat system. The corrosion resistance was found to increase with microcapsule addition. Thermogravimetric Analysis (TGA) indicated enhanced thermal stability of Tetra in the microcapsule.

Experimental characterisation and modelling of mechanical behaviour of microcapsules

Results of experimental characterisation and modelling of mechanical behaviour of microcapsules for self-sensing polymer composites are presented. Melamine–formaldehyde microcapsules were selected for this purpose. The average diameter, size distribution, and shell thickness of microcapsules were evaluated from scanning electron microscopy images. Compressive properties of the shell material were evaluated in several ways. AFM measurements allowed estimating stiffness and strength of a single microcapsule. In parallel, modelling of the mechanical behaviour of a single microcapsule was performed. Buckling of a thin-walled spherical shell under external pressure was considered and closed-form solution in a linear statement was obtained. The results of analytical calculations were compared with FEM modelling. Two demonstrator thin-shells with the radius of centimetre scale, hollow and filled with water, were tested in compression between two rigid plates. The results of their numerical analysis obtained by FEM models developed are in good agreement with experimental results.