Mechanical properties and thermal stability of double‐shell thermal‐energy‐storage microcapsules

Double‐shell‐structured microcapsules encapsulating phase‐change materials (micro‐PCMs) with an average diameter of 5–10 μm were successfully fabricated with a melamine–formaldehyde resin as the coating material. The mechanical properties of the obtained piled micro‐PCMs, tested under compression, were evaluated with a pressure sensor. Typical stress–strain curves showed that both the single‐shell‐ and double‐shell‐structured microcapsules had yield points and maximum point pressures. The morphological changes in the shell surface confirmed the existence of yield points by scanning electron microscopy. When the pressure was beyond the yield point, the microcapsules showed conventional plastic behavior, and the double‐shell structure was more mechanically stable than the single‐shell one. Differential scanning calorimetry analysis results revealed that the properties of the phase‐change materials experienced no variation after coating with a single‐shell‐ or double‐shell‐structured polymer. Thermogravimetric analysis showed that the double‐shell‐structured micro‐PCMs experienced a weight loss of only about 5% from 86.3 to 232°C but did so more rapidly from 232 to 416°C. Thermoregulation was determined with periodical heating and cooling tests. The data showed that the micro‐PCMs changed temperature in a narrow range of 20–25°C with a time lag of 20 min to reach the maximum or minimum temperature in comparison with a reference temperature of 18–28°C. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 1295–1302, 2007     

Synthesis and thermal properties of phase‐change microcapsules incorporated with nano alumina particles in the shell

Novel phase‐change microcapsules with paraffin as core and melamine‐formaldehyde (MF) resin as shell were synthesized through in situ polymerization, in which nano alumina (nano‐Al₂O₃) particles were dispersed in the shell by mixing nano‐Al₂O₃ with MF prepolymer solution using the direct addition method (i.e., adding nano‐Al₂O₃ into the MF prepolymer solution directly) and the predispersed addition method (i.e., predispersing the nano‐Al₂O₃ homogenously in water under the assistance of dispersant and wetting agents before mixing with the MF prepolymer). Scanning electron microscope experiments demonstrated that the predispersed addition method yielded the microcapsules having the better dispersion and less self‐agglomeration of alumina, compared to the direct addition method. Fourier transform infrared spectroscopy, energy dispersive X‐ray spectroscopy, and electron backscatter diffraction imaging confirmed that the nano‐Al₂O₃ particles were successfully incorporated in the shell by the predispersed addition method. The phase change behavior of microcapsules incorporated with different contents (up to 12.7% relative to the microcapsule) of nano‐Al₂O₃ particles in the shell was investigated by differential scanning calorimeter. The results revealed that the encapsulation efficiency for this kind of novel microcapsules was >77% and the incorporation of nano‐Al₂O₃ in the shell affected the phase change temperature. Thermal gravimetric analysis indicated that the addition of nano‐Al₂O₃ improved the thermal stability of microcapsules remarkably. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Thermal and antimicrobial evaluation of cotton functionalized with a chitosan–zeolite composite and microcapsules of phase‐change materials

Antimicrobial cotton was developed with silver zeolites (SZs). Three different application approaches for the cotton surface functionalization were followed: (1) SZ alone, (2) SZ combined with chitosan film, and (3) chitosan–zeolite (CS–SZ) composites previously synthesized by a gelation process with sodium tripolyphosphate. All finishes were applied to the textile materials through conventional pad–dry–cure processes, and the obtained results were then compared. The resulting materials were characterized with Fourier transform infrared spectroscopy, scanning electron microscopy with energy‐dispersive X‐ray spectroscopy, differential scanning calorimetry, IR thermography, and contact angle measurements, and the antimicrobial activity was evaluated. The results suggest that the application of CS–SZ should be recommended for the production of textiles with antibacterial properties because the materials showed activity against Escherichia coli, Staphylococcus aureus, Candida albicans, and Trichophyton rubrum. In addition, the finishing can be combined with the application of microcapsules of phase‐change materials to obtain textiles with thermoregulating properties. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46135.     

The effect of synthesis condition on physical properties of epoxy‐containing microcapsules

The physical properties of microcapsules are largely influenced by the synthesis conditions such as weight ratio of core/shell material, agitation rate, reaction time, and different emulsifier. Different synthesis condition would lead to different property. It is an important issue for application in composites that require self‐healing microcapsules possessing rough surface morphology, less adhesion, less core material permeability, appropriate diameter and core content, and adequate shell thickness. The properties of microcapsules influenced by the synthesis conditions were investigated systematically in this article. According to orthographic factorial design, the most influencing factor on microcapsule's yield, core material, average shell thickness and average diameter, are concluded, respectively. The synthesis parameters when the epoxy‐containing microcapsules exhibit the optimum properties are concluded: 1.4 : 1 for the weight ratio of core/shell material, 250 rpm for the agitation rate, 3 h for the reaction time and 1.5% content for the emulsifier DBS. The chemical structure of resultant microcapsules is confirmed by FT‐IR, and core material of microcapsule exhibits reactivity through DSC measurement. Subsequently, the microcapsules are characterized by SEM, OM, and contact angle experiment so as to provide parameters of microcapsule's physical properties for making binary self‐healing materials. As a result, the resultant microcapsules are suitable for fabricating self‐healing materials. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Preparation of phase change materials microcapsules by using PMMA network‐silica hybrid shell via sol‐gel process

Encapsulation of phase change materials (PCM) using a poly(methyl methacrylate) network‐silica hybrid as the shell material has been developed. n‐Octadecane melted at 28°C was used as PCM. Based on the suspension polymerization process, the microcapsules were prepared successfully by mixing and by the reaction of ethylene glycol dimethacrylate with precopolymer solution with tetraethoxysilane (TEOS), whose resultant microcapsules had higher latent heat (ΔH = 151 J/g) than those without TEOS (ΔH = 88.3 J/g). The average size of the PCM microcapsules was about 10 μm. The silica content, n‐octadecane content, and latent heat of microcapsules were changed with varying ageing conditions, ageing time, and temperature. The highest amount of latent heat (ΔH = 178.9 J/g) and n‐octadecane content (73.3%) of the microcapsule were obtained when the inorganic/organic ratio of the microcapsule was 5%. It was difficult to increase n‐octadecane content (74% to 55.7–67.9%) and latent heat (180.5 J/g to 135.9–165.7 J/g) of the microcapsules by introducing different functional groups of coupling agents. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Poly(ethylene glycol)/poly(methyl methacrylate) blends as novel form‐stable phase‐change materials for thermal energy storage

In this study, we focused on the preparation and characterization of poly(ethylene glycol) (PEG)/poly(methyl methacrylate) (PMMA) blends as novel form‐stable phase‐change materials (PCMs) for latent‐heat thermal energy storage (LHTES) applications. In the blends, PEG acted as a PCM when PMMA was operated as supporting material. We subjected the prepared blends at different mass fractions of PEG (50, 60, 70, 80, and 90% w/w) to leakage tests by heating the blends over the melting temperature of the PCM to determine the maximum encapsulation ratio without leakage. The prepared 70/30 w/w % PEG/PMMA blend as a form‐stable PCM was characterized with optical microscopy and Fourier transform infrared spectroscopy. The thermal properties of the form‐stable PCM were measured with differential scanning calorimetry (DSC). DSC analysis indicated that the form‐stable PEG/PMMA blend melted at 58.07°C and crystallized at 39.28°C and that it had latent heats of 121.24 and 108.36 J/g for melting and crystallization, respectively. These thermal properties give the PCMs potential LHTES purposes, such as for solar space heating and ventilating applications in buildings. Accelerated thermal cycling tests also showed that the form‐stable PEG/PMMA blend as PCMs had good thermal reliability and chemical stability. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Functionalization of wool fabric with phase‐change materials microcapsules after plasma surface modification

The use of microcapsules has increased in several different areas, namely, textile applications. They have been used as a possible means of introducing new properties, namely, in medical care by antibiotics, skin moisturizers, and other drugs and for thermal comfort. In this study, we examined the influence of dielectric barrier discharge (DBD) plasma treatment on the adhesion of phase‐change material (PCM) microcapsules on wool fabric. Several experimental techniques were used to evaluate the wool surface modification after plasma treatment and the influence of the microcapsules' resistance to washing conditions, namely, the determination of the static and dynamic contact angles, surface energy, and adhesion work; X‐ray photoelectron spectroscopy; Fourier transform infrared spectroscopy; differential scanning calorimetry; and scanning electron microscopy. Chemical and physical characterization of the wool fiber in the fabric confirmed significant surface modification. The plasma treatment greatly increased the hydrophilicity, surface energy, and adhesion work of the wool fabric; this proved that more microcapsules were adsorbed on the fabric and more microcapsules remained on the fabric surface after the washing procedures. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Preparation and characterization of microcapsules containing capsaicin

With urea‐formaldehyde (UF) resin as walls and capsaicin as core substances, microcapsules were prepared based on in situ polymerization process. The morphology and size distribution of the microcapsules were analyzed by Fourier transform infrared spectroscopy, laser particle size analyzer, and scanning electron microscopy. The microcapsulated capsaicin (MC) agents had a mean diameter of about 30–50 μm. Moreover, the thermal properties of the MC agents were measured by differential scanning calorimetry and thermogravimetric analysis. It was demonstrated that the melting point and thermal stability of the MC agents were greatly improved compared with that of the uncovered capsaicin, which were caused by the encapsulating crosslinked UF resin over the surface. The shell formation mechanism and the effects of the process conditions such as U/F ratio, shearing force, and acidification time on the particle size of the MC agents were discussed. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Development of thermoregulating textile materials with microencapsulated phase change materials (PCM). II. Preparation and application of PCM microcapsules

Melamine–formaldehyde microcapsules containing eicosane were prepared by in situ polymerization. The characterization of the microcapsules, including the particle size and size distribution, morphology, thermal properties, and stability, was carried out. The prepared microcapsules were added to polyester knit fabrics by a conventional pad–dry–cure process to develop thermoregulating textile materials. The morphology, thermal properties, and laundering properties of the treated fabrics were also investigated. The microcapsules were spherical and had melamine–formaldehyde shells containing eicosane. The microcapsules were strong enough to secure capsule stability under stirring in hot water and alkaline solutions. The heat storage capacity increased as the concentration of the microcapsules increased. The thermoregulating fabrics had heat storage capacities of 0.91–4.44 J/g, which depended on the concentration of the microcapsules. The treated fabrics retained 40% of their heat storage capacity after five launderings. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 2005–2010, 2005     

Thermal and morphological studies of chemically prepared emeraldine‐base‐form polyaniline powder

In this study, emeraldine base (EB)‐form polyaniline (PANI) powder was chemically prepared in 1M HNO₃ aqueous solution. The thermal characteristics and chemical structures of this powder were studied by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), and X‐ray diffraction (XRD). A polarizing optical microscope was also used to examine the crystalline morphology of this sample. The results indicated that the EB‐form PANI powder had a discernible moisture content. Moreover, in the first run of DSC thermal analysis, the exothermic peak at 170–340°C was due to the crosslinking reaction occurring among the EB‐form PANI molecular chains. FTIR and XRD examinations further confirmed the chemical crosslinking reaction during thermal treatment. TGA results illustrated that there were two major stages for weight loss of the EB‐form PANI powder sample. The first weight loss, at the lower temperature, resulted from the evaporation of moisture. The second weight loss, at the higher temperature, was due to the chemical structure degradation of the sample. The degradation temperature of the EB‐form PANI powder was around 420–450°C. The degradation temperature of emeraldine salt (ES)‐form PANI powder was lower (around 360–410°C) than that of the EB form (around 420–450°C). From the TGA results, I roughly estimated that 2.74 aniline repeat units, on average, were doped with 1 HNO₃ molecule in the ES‐form PANI. I found a single crystalline morphology of EB‐form PANI, mostly like a conifer leaf. More complex, multilayered dendritic structures were also found. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2142–2148, 2003     

Properties of poly(urea‐formaldheyde) microcapsules containing an epoxy resin

Physical properties of urea‐formaldehyde microcapsules containing an epoxy resin are presented and discussed. Microcapsules were prepared by in situ polymerization of monomers in an oil‐in‐water emulsion. Differential scanning calorimetry, thermogravimetric analysis, and scanning electronic microscopy were applied to investigate thermal and morphological microcapsule properties. Microencapsulation was detected by means of FTIR and Raman techniques. It was found that the amount of encapsulated epoxy resin as well as the extent of urea‐formaldehyde polymerization depends on the reaction temperature and the stirring speed. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Improvements in the thermal conductivity and mechanical properties of phase‐change microcapsules with oxygen‐plasma‐modified multiwalled carbon nanotubes

The thermal properties and mechanical properties are the key factors of phase‐change microcapsules (microPCMs) in energy‐storage applications. In this study, microPCMs based on an n‐octadecane (C18) core and a melamine–urea–formaldehyde (MUF) shell supplemented with O₂‐plasma‐modified multiwalled carbon nanotubes (CNTs) were synthesized through in situ polymerization. Meanwhile, two different addition methods, the addition of modified CNTs into the emulsion system or into the polymer system, were compared and examined. Scanning electron microscopy micrographs showed that the microPCMs were spherical and had a broadened size distribution. Fourier transform infrared testing demonstrated that the modified CNTs did not affect C18 coated by MUF resin. The results indicate that the thermal conductivity and mechanical properties of the microPCMs were remarkably improved by the addition of a moderate amount of modified CNTs, but the heat enthalpy and encapsulated efficiency decreased slightly. Moreover, the thermal conductivity and mechanical properties of microPCMs modified with CNTs directly added to the polymer system were superior to those with CNTs added to emulsion system. In particular, when 0.2 g of modified CNTs were added to the polymer system, the thermal conductivity of the microPCMs was improved by 225%, and the breakage rates of the microPCMs at 4000 rpm for 5, 10, and 20 min decreased by 74, 72, and 60%, respectively, compared with that of the microPCMs without modified CNTs. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45269.     

The role of radical polymerization in the production of thermoregulating microcapsules or polymers from saturated and unsaturated fatty acids

The microencapsulation of linoleic (LinA), oleic, erucic, and palmitic acids (PAs) from styrene and divinylbenzene were studied by using the suspension‐like polymerization technique. All materials exhibited a spherical shape, with a particle size between 166 and 416 μm. The phase change material (PCM) content decreased with the presence of double bonds in the fatty acid molecule. The thermal energy storage (TES) capacity of the microcapsules (MC) containing saturated PA was the highest (123.30 J g^?1). Whereas, the lowest TES capacity was observed for the LinA. TES capacity values from unsaturated fatty acid materials and the high particle yield indicated that these kinds of acids played two different roles, as PCM and also as monomers, in the radical polymerization processes. At high initiator concentrations, the unsaturated fatty acids were observed to react. This was confirmed by Fourier transform infrared where the peak assigned to the C?C bond disappears in the spectrum of MC. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45970.     

Perfume‐containing polyurea microcapsules with undetectable levels of free isocyanates

Core‐shell polyurea microcapsules with a 40% fragrance load were prepared by interfacial polymerization of guanidine and a technical polyisocyanate prepolymer containing mainly the biuret trimer derived from hexamethylene di‐isocyanate (HDI). Residual free polyisocyanates were still present at a level slightly above 100 mg NCO functional group per kg as determined by liquid chromatography hyphenated with tandem mass spectrometry of HDI and of its biuret trimer. This level was decreased by a factor of about 10 when the polymerization process was allowed to proceed for a longer time and by a factor of about 500 when guanidine or NaOH were added to the microcapsule suspension to act as scavengers. In these cases, polyisocyanate conversion was observed to proceed for about one month when the microcapsules were stored at room temperature before reaching a plateau at a level below 1 mg NCO/kg. Overall, ammonia was the most efficient polyisocyanate scavenger as no residual HDI biuret trimer and only less than 2 μg NCO/kg as HDI were detected at the end of the process, a level which had dropped below the limit of detection of 0.25 μg NCO/kg after about 40 days of aging at room temperature. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Effect of glass transition temperature on heat‐responsive gas bubbles formation from polymers containing tert‐butoxycarbonyl moiety

Various types of polymers containing tert‐butoxycarbonyl (BOC) moiety as the typical protecting group of functional moieties have been used for the design of stimuli‐responsive polymer materials. In this study, we investigated the heat‐responsive deprotection behavior of BOC‐containing polymers obtained by radical polymerization of 4‐(tert‐butoxycarbonyloxy)styrene (BSt) and copolymerizations of BSt with styrene and methyl acrylate. The deprotection of BOC groups accompanying the evolution of isobutene and carbon dioxide as gaseous products was monitored by thermogravimetric analyses at different temperature circumstances; that is, on heating at a rate of 10 °C/min and under isothermal conditions at various temperatures. The deprotection resulted in a significant decrease in the transmittance of visible light due to the formation of a large number of gas bubbles, that is, foaming, in the polymer films when a heating temperature was close to the glass transition temperature of the used polymer. The potential of BOC‐containing polymers was also evaluated as the heat‐responsive adhesive polymers for dismantlable adhesion. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46252.     

Preparation,characterization, and prominent thermal stability of phase‐change microcapsules with phenolic resin shell and n‐hexadecane core

Microcapsules with phenolic resin (PFR) shell and n‐hexadecane (HD) core were prepared by controlled precipitation of the polymer from droplets of oil‐in‐water emulsion, followed by a heat‐curing process. The droplets of the oil phase are composed of a polymer (PFR), a good solvent (ethyl acetate), and a poor solvent (HD) for the polymer. Removal of the good solvent from the droplets leads to the formation of microcapsules with the poor solvent encapsulated by the polymer. The microstructure, morphology, and phase‐change property as well as thermal stability of the microcapsules were systematically characterized by scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FTIR), differential scanning calorimety (DSC), and thermogravimetric analysis (TGA). The phase‐change microcapsules exhibit smooth and perfect structure, and the shell thickness is a constant fraction of the capsule radius. The initial weight loss temperature of the microcapsules was determined to be 330°C in N₂ and 255°C in air, respectively, while that of the bulk HD is only about 120°C both in air and N₂ atmospheres. The weight loss mechanism of the microcapsules in different atmosphere is not the same, changing from the pyrolysis temperature of the core material in N₂ to the evaporation of core material caused by the fracture of shell material in air. The melting point of HD in microcapsules is slightly lower than that of bulk HD, and a supercooling was observed upon crystallization. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Solid–solid phase‐change materials based on hyperbranched polyurethane for thermal energy storage

A hyperbranched polyol (HBP) was synthesized with poly(ethylene glycol) (PEG) as the core molecule and 2,2‐bis(hydroxymethyl) propionic acid as the chain extender. Then, a series of hyperbranched polyurethane phase‐change materials (HP‐PCMs) with different crosslinking densities was synthesized with isophorone diisocyanate and HBP as a molecular skeleton and PEG 6000 as a phase‐change ingredient. ¹H‐NMR, gel permeation chromatography, and Fourier transform infrared spectroscopy confirmed the successful synthesis of the HBP and HP‐PCMs. The polarization optical microscopy and wide‐angle X‐ray diffraction results show that the HP‐PCM exhibited good crystallization properties, but the crystallinity was lower than that of PEG 6000. The analysis results from differential scanning calorimetry indicated that the HP‐PCMs were typical solid–solid phase‐change materials with suitable phase‐transition temperatures. In addition, HP‐PCM‐3, with an appropriate degree of hyperbranched structure, possessed the highest thermal transition enthalpy of 123.5 J/g. Moreover, thermal cycling testing and thermogravimetric analysis showed that the HP‐PCMs exhibited good thermal reliability and stability. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45014.     

Acrylic bone cements modified with poly(ethylene glycol)‐based biocompatible phase‐change materials

The high polymerization temperature of acrylic bone cements used in hip replacement implantation may cause thermal necrosis of surrounding tissues. In order to reduce the polymerization temperature, acrylic bone cement has been modified with a biocompatible polymeric phase‐change material (PCM) based on poly(ethylene glycol) (PEG) of different molecular weights and stabilized with potato starch. Structural and morphological studies were performed, and the thermal and mechanical properties were investigated. The incorporation of PEG‐based PCM led to a decrease in the polymerization temperature of bone cement from 70 °C for unmodified cement to 58 °C for modified cement. Modified cement materials were stable in incubation tests, although acoustic analysis data revealed a decrease in propagation speed after incubation, which indicates formation of material defects (pores, cracks, voids, etc.) due to water activity. However, in the regeneration process, these defects can be filled by freshly grown bone tissue leading to better incorporation of bone cement replacements into tissue. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43898.     

Detailed experimental and theoretical investigation of the thermomechanical properties of epoxy composites containing paraffin microcapsules for thermal management

Microencapsulated phase change materials (PCMs) are being increasingly employed as functional fillers in polymer matrices for thermal management. Therefore, the effect of PCM microcapsules on the rheological, microstructural, thermal and mechanical properties of the host polymer matrices must be understood. This work concerned the preparation of epoxy polymers filled with PCM microcapsules (MC) in various concentrations and their subsequent characterization. The MC phase increases the viscosity of epoxy before curing, thereby reducing castability and hindering elimination of air bubbles. The latter was reflected in an increase in porosity for highly filled compositions, as elucidated by density measurements and microscopic investigation. SEM micrographs showed that the adhesion between the capsule shell and the epoxy matrix was not optimal. The interfacial weakness and the intrinsic low stiffness and strength of MC caused a reduction in mechanical properties, as evidenced by the Nicolais-Narkis and Pukanszky models. On the other hand, at zero or low deformation levels, the interface presents no gaps and is able to transfer load and heat, as demonstrated by the data of elastic modulus, modeled with the Halpin-Tsai and Lewis-Nielsen models, and those of thermal conductivity, in excellent agreement with the Pal model. POLYM. ENG. SCI., 2020. © 2020 Society of Plastics Engineers     

Preparation of polyamide 6/polystyrene quasi‐nanoblends by diffusion and subsequent polymerization of styrene in water‐sorbed polyamide 6 pellets

Polyamide 6 (PA6)/polystyrene (PS) blends with an average particle size of 103 nm were prepared by diffusion and subsequent polymerization of styrene in water‐sorbed PA6 pellets. The pretreatment of PA6 pellets in hot water is prerequisite for successful styrene diffusion. The diffusion process involves replacement of free water in the pellets by styrene, and should be carried out in neat styrene medium to provide concentration gradient between inside and outside of the pellets. The polymerization step was carried out in water medium with benzoic peroxide as the initiator. The diametrical distribution of PS in the blend pellets was investigated by Micro‐FTIR, and molecular weight of PS was measured by GPC. DSC measurements showed that the diffusion and polymerization of styrene occur in the amorphous regions of PA6 where the pre‐sorbed water locates. PA6/PS quasi‐nanoblends reported in this work cannot be obtained by conventional methods. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44554.