Synthesis of a paraffin phase change material microencapsulated in a siloxane polymer
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The coemulsification method suitable for the formulation of microcapsules of n-eicosane coated with a polysiloxane is developed. This method allows to synthesize core–shell microcapsules of paraffin which have the shape of spheres or distorted spheres and are designed for the use as phase change materials. The microcapsules are formed in aqueous phase by the precipitation of n-eicosane together with modified polyhydromethylsiloxane from a common solvent which is miscible with aqueous media. The polysiloxane is modified by the attachment of silylvinyl and alkoxy functions before coemulsification with the paraffin. It also contains the Pt(0) Karstedt catalyst. The microcapsules formed by coemulsification are stabilized by the in situ cross-linking of the polysiloxane shell. The shell is additionally modified by the in situ generation of silanol groups which provide colloidal stabilization of microspheres in aqueous phase. Microcapsules were studied by DSC, SEM, optical polarized microscope, and by thermooptical analysis (TOA).
KeywordsPCM microcapsules Microencapsulation Microencapsulated phase change materials Polyhydromethylsiloxane Microcapsules Karstedt catalyst
There has been a growing interest in phase change materials (PCM), which are converted from solid to liquid and from liquid to solid phase with a large heat effect. They are commonly used for thermoregulation, thermal energy storage, and cooling. Among many other applications they are used by textile industry for the thermoregulation of clothing [1, 2, 3, 4, 5]. In clothes, the PCM, which has a suitably matched phase change temperature, is soothing the effect of outside thermal peaks by the absorption and release of thermal energy. It gives the human body a feeling of thermal comfort and protects people working in adverse temperature conditions against thermal shocks. Paraffins, linear chain saturated hydrocarbons, with the melting and crystallization temperatures quite similar to the temperature of the human body (e.g., eicosane), are often used as PCM [6, 7, 8]. Importantly, they have a high latent heat of fusion which gives them high heat capacities. Their fusion temperature depends on the hydrocarbon chain length, so it may be easily adjusted to the needed temperature range. Moreover, paraffins are chemically and thermally stable and physiologically neutral.
Although PCMs are used in various forms, such as bulk materials, sheets, and particles dispersed in matrices, the most convenient form of their use in textile industry is core–shell microcapsules [9, 10]. Microcapsules can be easily dispersed in materials or attached to their surfaces. The rate of heat exchange with an encapsulated PCM is high due to its large surface to volume ratio. Moreover, microcapsules do not change the outer appearance of clothing and its characteristics, such as softness, breathability, and color. Melamine [11, 12, 13, 14, 15] or urea [16, 17, 18] resins cross-linked by formaldehyde are frequently used as the encapsulating material. Since there is a tendency to eliminate carcinogenic formaldehyde from the manufacturing of products used in contact with human body, our attention was turned to polysiloxanes as candidates for encapsulants for PCMs. These polymers are attractive because they are physiologically neutral, chemically and thermally stable, and are resistant to oxygen, water, and water vapor. They are sufficiently mechanically strong and elastic to withstand the volume change and deformation of PCMs during the phase change. The thermomechanical damage of the PCM microcapsule shell due to its rigidity may be a problem , and in some cases, soft segments are introduced into encapsulating polymer with purpose to prevent the shell breakage [19, 20].
Polysiloxane particles have been used as drug carriers for drug-controlled release [21, 22], carriers of metal catalysts [23, 24], dyes , and magnetic nanoparticles . These particles are often named capsules but they do not have core–shell structure. They have form of microsphers in which other materials are dispersed. This type of particles does not fit for PCMs as the dispersion decreases paraffin thermal capacity. Organotrialkoxysilanes were used for the formation of polysilsesquioxane coating on particles by a sol–gel interfacial process [27, 28]. These coatings are rigid and may be cut or broken by sharp edges of crystals formed in the phase change process. Miniemulsion technique was recently used to encapsulate some materials with polysiloxanes. This method leads to the nano-sized core–shell particles [29, 30], which are not suitable for PCMs.
The encapsulation of a paraffin particle with polysiloxane to form a core–shell micron-sized structure is not an easy problem. Most of the encapsulation procedures are performed in aqueous systems. Polysiloxane monomers are soluble in paraffins and insoluble in water which makes the encapsulation of paraffin particles by an emulsion polymerization difficult. Coating of paraffin particles with a hydrophobic polysiloxane is also hindered because this polymer has a tendency to form a separate microphase in aqueous systems. Our attempts to use a polysiloxane with hydrophilic tetraalkylammonium chloride groups pendant to the polymer chain were unsuccessful as well. Unexpectedly we found that stable microcapsules with n-eicosane core and polysiloxane shell may be obtained when a modified polyhydromethylsiloxane is subjected to coemulsification with the paraffin in an aqueous system, with the in situ cross-linking of the encapsulant and with simultaneous additional modification of the formed particle shell. Our method is based on particular activity of the Karstedt catalyst (platinum Pt(0) complex with 1,3-divinyl-1,1,3,3-tetramethyldisiloxane) which may simultaneously act as the catalyst of four reactions involved in the encapsulation process, i.e., hydrosilylation , alcoholysis of hydrosilane , hydrolysis of SiH and SiH condensation with SiOH .
Polyhydromethylsiloxane (PHMS) with trimethylsiloxane end groups was a product of ABCR Gmbh under name HMS 991 having viscosity 15–25 cSt corresponding to Mn = 1.5 × 103 to 2.0 × 103 g/mol; 1,3-divinyltetramethyldisiloxane (DVTMDS) with declared purity 97 % was provided also by ABCR poly[vinyl alcohol] (PVA) with declared purity 99.5 % and Mn 7.2 × 104 g/mol was purchased from Polskie Odczynniki Chemiczne (POCH); isopropanol with declared purity 99.5 % was also obtained from POCH; tetrahydrofurane from CHEMPUR was p.a. grade dioxane with declared purity 99.8 % was purchased from CHEMPUR; n-eicosane with declared purity 99 % was from Alfa Aesar Gmbh. All these chemicals were used without additional purification; however, their purity was confirmed by the gas chromatography analysis. Platinum complex 20 wt% Pt(0) was kindly offered by Momentive Performance Materials Leverkusen.
Thermal characteristics: differential scanning calorimeter (DSC) thermograms were registered with a DSC 2920 Temperature Modulated TA Instrument at the rate of temperature change 5 K/min and using ca. 5-mg samples. Thermogravimetric measurements were performed using a HIRES TGA 2950 Thermogravimetric Analyzer TA Instruments. Thermooptical studies were performed using a METTLER TOLEDO FP82HT HOT stage with control processor FP90.
Solid state 29Si NMR spectra were registered with a DSX 400 Bruker spectrometer. The spectra were acquired with cross-polarization, at 59.627 MHz applying 90-μs pulses, 6-s pulse delay, and 3-ms contact time, with samples in 4.0-mm zirconia rotors spinning at 8 kHz. The peak positions were referenced to the signal of Q8M8.
SEM images were taken with a JEOL JSH 5500 LV in high-vacuum mode at the accelerated voltage of 10 kV. Samples were coated with a fine gold layer, about 20 nm thick using ion coating JEOL JFC 1200 apparatus. Optical microscopy observations of microcapsules were done using microscope optical with polarizer Nicon Eclips E400 POL.
Generation of microcapsules with n-eicosane core
The solution of 16.5 g of PHMS with 3.1 g of DVTMDS in 25 mL of tetrahydrofurane (THF) was heated to 45 °C and 0.025 g of a Pt(0) catalyst solution was introduced. An increase of viscosity of the solution was observed. The solution was stirred for 20 min at 45 °C and then introduced to a reactor in which the solution of n-eicosane 40 g in 110 mL of isopropanol was stirred at 45 °C. The resulting solution was stirred for additional 5 min and thermostated at 45 °C. Then the solution was quickly introduced to the 150 mL of water thermostated at 45 °C containing the 0.55 g of dissolved PVA. The mixture was homogenized for 25 s using a high-speed MPW-120 homogenizer set to 4,000 rpm. The formed emulsion was introduced to a 5-L reactor equipped with an anchor stirrer in which 1.85 L of the aqueous solution of the 6.55 g of PVA was stirred at 45 °C. Stirring was continued for 70 h after which microcapsules were separated by centrifugation. The isolated microcapsules were carefully washed with water to remove surfactant from their surfaces. Their shape did resemble distorted spheres and the yield of microcapsules with the linear size in the range between 2 and 12 μm was 70 %. Temperatures of fusion and crystallization were 37.4 and 30.1 °C respectively. The heat of fusion was 159 J/g.
Results and discussion
Preparation of microcapsules
The attachment of DVTMDS to the polysiloxane chain in the first step is necessary to prevent the penetration of this compound into the paraffin phase during the coemulsification and stabilization steps. However, the pendant vinyl groups may undergo subsequent hydrosilylation, which finally would results in the polymer cross-linking. This hydrosilylation is not desirable in the initial step, so at a suitable moment the reaction is slowed down by the dilution of the system with isopropanol solution of paraffin to give the time to perform further operations needed to form the polysiloxane coating on paraffin core before the cross-linking occurs.
In order to determine the moment at which the first step of the hydrosilylation process should be terminated, this reaction was parallelly monitored by the tracking of the variation of the viscosity of the reacting mixture. In the initial stage, when reaction 1 dominates, the viscosity increases slowly. The consecutive hydrosilylation of the second vinyl group is accompanied with a more rapid increase in the viscosity, which approaches infinity at gel point. At the half time of the hydrosilylation reaction needed to achieve gel point, i.e., well before the viscosity rapidly increases, the reacting mixture is added to solution containing the paraffin dissolved in a significant amount of isopropanol or in a mixture of isopropanol with dioxane or THF.
The concentrated solution of the modified PHMS with paraffin is thermostated at a temperature above the paraffin melting point and is mixed, with a limited amount of aqueous solution of PVA used here as a surfactant. This operation is performed using a high-speed homogenizer, at a temperature above the melting point of paraffin. An emulsion of microdroplets of liquid paraffin embedded in polysiloxane is formed. The emulsion is unstable as the microcapsules are not fully formed and the encapsulant has not yet been cross-linked. The further physical and chemical processes are carried out after dilution of the emulsion by the PVA in water solution, having the same PVA concentration and the same temperature as the water used in the emulsification process.
It should be mentioned that the Karstedt catalyst has been used so far mostly for the catalysis of hydrosilylation . Its application for the catalysis of other reactions is very seldom, see references [32, 33]. In the process described here, the Pt(0) complex, besides hydrosilylation, catalyzes also three other above-mentioned reactions. All of them are very important for the successful synthesis of the microcapsule shells.
The isolation of microcapsules may be performed by sedimentation under gravitational forces or facilitated by centrifugation or by filtration. The density of the microcapsules is close to the density of water, which makes difficult their isolation by sedimentation. The presence of a submicron fraction of microparticles may pose a problem during the filtration. The microcapsules, after their isolation from the aqueous system, are subjected to freeze-drying. The yield of microcapsules was about 70 %. The optimal weight content of the polysiloxane coating was 33 %.
Characterization of microcapsules
29Si MAS NMR spectra were analyzed with the purpose to obtain information on the chemical structure of the microcapsule shells. A spectrum of n-eicosan-containing microcapsules is shown in Fig. 2. The dominating peak at −57 ppm is that of silicon in the polymer chain bearing the hydroxyl group. A small signal at −59 ppm of silicon atom with bound izopropoxyl group is superimposed on the right wing of the silanol signal. The silicon in the polymer chain linking it to another chain by carbosiloxane and by siloxane bridges give resonances at −21 and −66 ppm, respectively. Rather, a small number of the unreacted SiH groups remains in the polymer giving signal at −37 ppm. The polysiloxane shells contain a great number of silanol groups which are present in the polymer bulk. The most important are those which are located on the shell surface because they make surface of microcapsules hydrophilic, which helps to redisperse them in aqueous systems after removal of surfactant.
The exothermal peak of crystallization of n-eicosane is splitted to two and ranges from 34 to 25 °C with maxima at 31 and 30 °C. The splitting comes from the polymorphism of n-eicosane .
Thermogravimetric studies showed that the onset of the n-eicosane microcapsule thermal decomposition is 156 °C. At this temperature, cracking of the silicone coating of microsphere begins and the maximum of this process is observed at 213 °C. Finally, almost all core is lost by evaporation at about 240 °C. Polysiloxane shell undergoes restructurization as the sample temperature is increased and relatively large amount of residue is generated (about 33 wt% at 900 °C). A high content of the residue is due to a high degree of cross-linking of the polysiloxane, which prevents its depolymerization and makes easier its transformation into the SiCO ceramics.
Synthesis of paraffin encapsulated in polysiloxane
PCM (quantity) g
Catalyst Pt/capsulea, mol/g
Time of cross-linker addition, min
Heat of melt, J/g
Melt. temp. (range), °C
Cryst. temp. (range), °C
Mean size of capsule, μm
9.7 × 10−7
7.8 × 10−7
Isoprop. + THF 5:1 v/v
4.3 × 10−7
Isoprop. + dioxane 1:1 v/v
3.2 × 10−7
The coemulsification in aqueous phase of n-eicosane with polyhydromethylsiloxane having pendant vinyl groups may lead to the formation of core–shell microparticles in which the paraffin is embedded in a polysiloxane shell. These particles are in situ stabilized by the polysiloxane cross-linking using the hydrosilylation process catalyzed by Pt(0) complex (Karstedt catalyst). This catalyst promotes also the competitive formation of silanol groups on the polysiloxane and their partial condensation with the SiH groups which additionally crosslinks the polysiloxanes shell.
The coemulsification is carried out in the temperature above the paraffin melting point using isopropanol or its mixture with THF or dioxane as common solvents for the paraffin-polysiloxane-catalyst system, which are miscible with water. These core–shell particles may serve as PCM microcapsules. They have a good thermal characteristic and withstand 50 fusion-crystallization cycles test.
The research was performed in the frame of project NANOMITEX POIG 01.03.01-00-004-08 “Functional nano and micro textile materials” sponsored by European Union supervised by Institute of Textile Industrie in Łódź, Poland. Authors are grateful to Jos Delis PhD from Momentive Performance Materials Leverkusen for the generous gift of the concentrated solution of the Pt(0) complex catalyst.
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