|Title of Invention
PHASE CHANGE MATERIAL CAPSULE AND PROCESS FOR PREPARING THE SAME.
|PHASE CHANGE MATERIAL CAPSULE AND INSITU POLYMERIZATION ROCESS FOR PREPARING THE SAME The present invention describes phase change material (PCM) capsule wherein the PCM is employed for thermoregulatory mechanism such as thermal storage, and thermal regulation. It finds applications in thermo-regulated textile. The PCM capsules of this invention are having better stability to thermal heat, solvent and reduced diffusion rate. More particularly, the capsules of the present invention have high core content and high encapsulation efficiency without compromising/scarifying the wall strength and or wall porosity. The core comprising of organic compounds such as hydrophobic aliphatic alkanes alcohol, or acids while the wall consists of natural or synthetic polymer exemplified by but not limited to polyurea, urea formaldehyde, melamine formaldehyde resins. The invention also discloses a process for preparing the PCM capsules. Particularly the capsules are prepared through in-situ polymerization or interfacial polymerization. The monomers used in the process may be such as amine,, alcohol,, and diisocynates, melamine-forrnaldehyde, urea- formaldehyde, melamine-urea-formaldehyde, and urea-resorcinol-formaldehyde.
|This invention relates to a Phase Change Material Capsule and In-situ Polymerization Process for Preparing the Same
FIELD OF INVENTION:
The invention relates to encapsulated phase change material (PCM) wherein the PCM is employed for thermoregulatory mechanism. Specifically, encapsulated phase change materials (PCMs) of this invention intend to perform functions such as thermal storage, and thermal regulation. It finds applications in thermo-regulated textile. Particularly, the invention relates to a PCM capsule having better stability to thermal heat. Specifically, the capsules of the present invention have better stability for solvent and reduced diffusion rate. More particularly, the capsules of the present invention have high core content and high encapsulation efficiency without compromising/scarifying the wall strength and or wall porosity. The core comprising of organic compounds such as hydrophobic aliphatic alkanes, alcohol, or acids, while the wall consists of natural or synthetic polymer exemplified by but not limited to polyurea, urea formaldehyde, melamine formaldehyde resins.
The invention also discloses a process for preparing the PCM capsules. Particularly the capsules are prepared through in-situ polymerization or interfacial polymerization. The monomers used in the process may be such as amine, alcohol, and diisocynates, melamine-formaldehyde, urea- formaldehyde, melamine-urea-formaldehyde, and urea-resorcinol-formaldehyde. Further, the polymerization process of this invention is cost effective, and efficient as it yields better product.
BACKGROUND OF THE INVENTION:
Encapsulation is a process of enclosing an active agent in a polymeric shell. The active agent can be in the form of a solid, a liquid, or even a gas, whereas, the wall material can be an organic polymer, hydrocolloid, sugar, wax, fat, metal, or inorganic oxide. The PCM useful for the textile applications are those that change phase from solid to liquid by absorbing large amount of latent heat from the environment and change phase from liquid to solid by releasing the same amount of heat to the environment at a transition temperature close to the body temperature.Suitable PCMs, that satisfy the major requirements of thermoregulation for application in
textile clothing, are n-octadecane, n-eicosane and lithium nitrate tri-hydrate. n-Octadecane is particularly important because its melting temperature Is very close to the comfort temperature of the body i.e. around 28-29°C, and it has a high latent heat of fusion of 240 J/g. However, this PCM must be encapsulated using a suitable technique that can provide high wall integrity and stability. This requirement of better wall integrity is unlike other microencapsulation processes used in pharmaceutical products, cosmetics and textile finishes, where micropores in the wall-membrane are necessary for slow release of drug, perfumes, finishing agent that are present as core.The relevant prior art known to the inventor with regard to the characteristics of the capsule include article that appeared in Colloid and Polymer Science (2002), 280(3), 260-266, titled "Microencapsulation of octadecane as a phase-change material by interfacial polymerization in an emulsion system" by Cho, Jeong-Sook; Kwon, Aehwa; Cho, Chang-Gi. The core content of the PCM capsules as disclosed in the said paper is 40 to 45%. According to the said paper Microcapsules containing phase-change material for thermal adaptable fiber application were synthesized and characterized. The microcapsules of about 1µm in diameter were prepared using an interfacial polycondensation method with toluene-2,4-diisocyanate (TDI) and diethylenetriamine (DETA) as monomers in an emulsion system. Octadecane was used as a phase-change material and NP-10, which is nonionic surfactant, was used as an emulsifier. To investigate the reaction ratio of monomers, microcapsules were synthesized with 3 g TDI and 0-4 g DETA. Polyurea microcapsules were formed not only by reaction with TDI and DETA, but also by reaction of TDI with hydrolyzed TDI at the interface. TDI was reacted with DETA in the wt. ratio of 3:1. NP-10 was reacted with TDI to form urethane. According to this paper, the efficiency of octadecane encapsulation increased as the core content decreased.CN1467260, titled Microcapsulated heat absorbing particle, describes a microcapsule-type heat absorbing particle able to be sprayed on the surface of object for absorbing a great deal of heat, is a (l-1000)-micron particle, which is composed of shell and core. Said shell is a high-molecular polymer chosen from melamine formaldehyde, urea-formaldehyde resin, PU, etc. Said core is a heat-absorbing substance chosen from straight-chain alkane, high-grade alcohol and organic acid.
A reference also can be made to Korean Patent No. KR 2002-11266 20020304 titled Preparation of thermostable microcapsules containing phase change materials and articles. According to this patent the method comprises steps of: adding 10 to 50 parts organic phase consisting of 0 to 70% nonpolar organic solvent, 10 to 90% polymerizable monomer phase change materials having alkyl group having 8 to 18 carbon atoms and 10 to 50% compound having two or more isocyanate groups to a solution in which 0.2 to 10 parts surfactant is dissolved into 100 parts water, and emulsifying the mixed solution as agitating the solution and organic phase; and injecting at least one compound selected from diethylenetriamine, ethylenediamine, triethylenetetramine and melamine solution and the compound having two or more isocyanate groups to the emulsified solution in a ratio of 100:20 to 300, and reacting the compounds with emulsified solution so that urea bond is formed.However, the core content and thermal stability is not explicitly mentioned. Further, the procedure does not specify time of addition of the second monomer i.e. diamine or triamines.
US 2004121072 Al. In accordance with this patent, the process for encapsulating a core component comprising paraffin compounds that can undergo phase transition, includes the steps of: (A) dissolving first monomers in the core component to form a first solution, the first monomers forming a first polymer; (B) dispersing the first solution into a second aqueous solution to form an emulsion; (C) adding the said emulsion to a third aqueous solution containing a precondensate of melamine-formaldehyde to form an encapsulation mixture characterized in that the first monomers are a polypropylene glycol with an average mol. wt. higher than about 400 and a polyisocyanate soluble in the core component. This patent claims addition of emulsion to the melamine-formaldehyde pre-polymer, maintaining melamine to formaldehyde ratio between 1:1.3 and 1:3, maintaining pH between 4 and 7. Further, the encapsulation was effected at 70°C.As can be seen from the above description, the information on the characteristics of the PCM capsule is rather scanty. The process for preparing the said capsules uses the melamine formaldehyde ratio in the range of 1:1.3 and 1:3.0. Preparation of prepolymer with lower ratio results in melamine formaldehyde resin of lower molecular weight that
influences the condensation of resin on hydrophobic core material. This in turn affects strength of shell wall. Thus, though the existing encapsulation processes may be quite effective each of them suffer from some drawbacks. Particularly, the shell wall becomes
more permeable when heated at about 130 C and above especially in dry state, which
may be mandatory for fabrics coating in the curing stages. Also, only a limited amount of core content can be encapsulated.The inventors after extensive research have arrived at a solution to this problem. They have found out that when the hexamethylolmelamine prepolymer, prepared using more than 6 times formaldehyde than melamine, is used the condensation rate gets regulated in advantageous manner due to formation of melamine-formaldehyde resin with higher molecular weight. This enables developing shell wall of superior characteristics. Further, the residual formaldehyde can be removed by any known methods to ensure safety of the final products.As herein before disclosed, the novel features of the present invention are:
1. Obtaining the reaction emulsion mixture by preparing emulsion inside the aqueous
solution of melamine-formaldehyde pre-polymer.
2. Maintaining the wall material composition at the formaldehyde to melamine (fm)
ratio of 6-8. Further this combination gave pre-polymer species having hexamethylol
melamine-formaldehyde that on curing gave completely crosslinked stable polymer
3. Regulating temperature profile to properly encapsulate the core material by wall
4. Preparing the emulsion at above the melting point of the core, 40°C in case of n-
octadecane, and initiating encapsulation at the same temperature followed by slowly
raising the temperature to 70°C while slowly bringing down the pH from 7 to 3.
5. Modifying drying and curing conditions to affect the slow but full condensation of
melamine and formaldehyde species. The drying was done at 40 °C for 12 hr and
final curing was carried out at 100 °C for 11/2 hrs.
6. Significant reduction or elimination of employing solvent such as cyclohexane in
case of polyurea capsules, if desired. This in turn makes the process cost effective as
well as high yielding and value added. This is because the reduction in the volume of
cyclohexane results in increased core contents and reduction in the cost associated with the amount of cyclohexane used.
7. Integrating speed of stirring and the manner in which monomer is added.
8. The obtained capsules through above modified processes had a high core content of
70 % and high stability to solvent wash. The capsules were stable > 100 °C without
any leakage and the process showed high encapsulation efficiency of >80%.
It may be pertinent to note that the novel features described herein above are illustrative and should not construe the scope of the invention.
OBJECTIVE OF THE INVENTION:
The main object of the present invention is to provide Phase Change Material Capsule.
The other object is to provide a phase change material (PCM) capsule wherein the PCM employed is for thermoregulatory mechanism. Specifically, encapsulated phase change materials (PCMs) of this invention intend to perform functions such as thermal storage, and thermal regulation.Another object is to provide a phase change material (PCM) capsule that finds applications in thermo-regulated textile in addition to the applications as herein after described.Yet other object is to provide a phase change material (PCM) capsule having better stability to thermal heat. More specifically, the capsules of the present invention have better stability for solvent and reduced diffusion rate.
Yet another object is to provide a phase change material (PCM) capsule that has high core content and high encapsulation efficiency without compromising/scarifying the wall strength and or wall porosity.The core comprising of organic compounds such as hydrophobic aliphatic alkanes, alcohols or acids, while the wall consists of natural or synthetic polymer exemplified by but not limited to polyurea, urea formaldehyde, melamine formaldehyde resins.The invention also discloses a process for preparing the PCM capsules. Particularly the capsules are prepared either through in-situ polymerization or interfacial polymerization. The monomers used in the process may be such as amines, alcohol, diisocynates, melamine-formaldehyde, urea- formaldehyde, melamine-urea-formaldehyde, and urea-resorcinol-formaldehyde.
Further, the polymerization process of this invention is cost effective, and efficient as it yields better product with varied applications.The application with regard to interfacial polymerization to produce PCM of the instant Invention has been divided out from this application.
STATEMENT OF INVENTION:
Accordingly the present invention provides a Phase Change Material (PCM) Capsule comprising of core essentially consisting of hydrophobic aliphatic alkanes with 8 to 25 carbon and wall consisting of polymeric material selected from urea formaldehyde, melamine formaldehyde resins, characterized in that the capsule has at least 50% core content, prepared at a maximum core to wall ratio of 4, formaldehyde to melamine F/M ratio more than 4, heat storage capacity of 160 J/g or more, particle size distributin of less than 10 microns, and heat, temperature, & solvent stability such as herein described.
According to one of the embodiments of this invention, the hydrophobic aliphatic alkanes, alcohol, or organic acids constituting core may be such as n-octadecane or other alkanes such as Dodecane, Tridecane, Tetradecane,Paraffin, Pentadecane, Hexadecane, Heptadecane, 1-Dodecanol, Octadecane, Nonadecane, Eicosane, 1-Tetradecanol, Heneicosane, n-Octacosane, n-Heptacosane, n-Hexacosane, n-Pentocosane, n-Tetracosane, n-Tricosane, n-Docosane , n-Homeiocosane, or any other inert organic molecules immiscible with water, preferably n-octadecane.According to other embodiment of this invention, the Particle size distribution may have particle size of less than 1 µ. preferably more than 100 nanometers and less than 10 µ further the mean particle size preferably is of 1-4 microns with stirring speed of 2500 and between 100 nanometer to 1µ with stirring speed of 2500 to 30000 during emulsification.
According to yet another embodiment of this invention, the core to wall ratio may be in the range of 1 to 3.5 preferably 1.8 to 2.5 the F/M ratio may be more than 6 preferably 6 to 8, more preferably 8.According to still other embodiment of this invention, the wall porosity may be of submicron size.In accordance with other aspect of this invention there is provided an insitu polymerization process for preparation of PCM capsule as disclosed herein above comprising contacting hydrophobic aliphatic alkanes with 8 to 25 Carbon atoms and melamine formaldehyde pre-polymer in a manner such as herein described to prepare emulsion and subject to regulated temperature at a pH below 4 to develop capsules followed by isolating, drying and curing the capsules.The contacting may be performed by (a) adding MF pre-polymer emulsion to PCM emulsion, (b) adding PCM emulsion to MF pre-polymer emulsion, or (c) mixing predetermined quantity of aqueous solution of FM pre-polymer and PCM solution in presence of anionic or nonionic surfactant protective colloid under vigorous stirring.The emulsion of MF pre-polymer used for addition to PCM emulsion may be maintained at pH greater than 7 preferably between 8.0 to 9.0 at in molten form and the addition was conducted slowly & at stirrer speed of more than 2000 rpm.The emulsion pH may be maintained at 2.5 to 3.0 and temperature at 60 to 90°C, preferably 70°C till completion of capsule formation.The drying may be effected at a temperature a minimum temperature of 40°C and curing is carried out in steps in the temperature range of 100°C -150 °C, preferably at 100°C for about 1 to Ihr and thirty minutes.The surfactant used may be anionic such as sodium lauryl sulphate and protective colloid used may be PVA.
DESCRIPTION OF THE INVENTION:
Characteristics of the wall are as follows:
Wall composition: melamine-formaldehyde or polyurea
Core: organic compounds such as hydrophobic aliphatic alkanes, alcohol, or organic
acids exemplified by n-octadecane or other alkanes such as Dodecane, Tridecane,
Tetradecane,Paraffin, Pentadecane, Hexadecane, Heptadecane, 1-Dodecanol,
Octadecane, Nonadecane, Eicosane, 1-Tetradecanol, Heneicosane, n-Octacosane, n-
Heptacosane, n-Hexacosane, n-Pentocosane, n-Tetracosane, n-Tricosane, n-Docosane ,
n-Homeiocosane, or any other organic molecules immicible with water and inert to
encapsulating compounds, preferably n-octadecane.
Particle size distribution has mean particle size of 3-4 microns when stirring speed of
2500 rpm is used and of 10 microns.
Heat storage capacity of about 160 J/g or higher; with nearly no super-cooling
Core content: 70% or higher.Wall porosity: Submicron size which allows n-octadecane to diffuse out in liquid bath
such as cyclohexane, acetone, or hot water is less than 10% of core content in 10 minutes
of washing cycle.
The encapsulation may be performed either by interfacial or insitu polymerization as
Interfacial polymerization using diisocynate and diamines (polyurea capsules)
Microencapsulation of phase change material (PCM) n-octadecane was carried out by interfacial polymerization technique using core and bulk monomers as toluene-2, 4-diisocyanate (TDI) and diethylene triamine (DETA), respectively. The bulk monomer that can be employed may be Diethylenetriamine (DETA), Ethylene Diamine (EDA), Triethylenetetramine (TETA), and Hexamethylene diamine (HMDA) and core monomer may be selected from Toluene Diisocyanate (TDI), and Methylene Diisocyanate (MDI). Cyclohexane was used as the solvent for TDI and n-octadecane, which formed the oil phase. The effect of core to monomer (CM) ratio and PCM to cyclohexane (PC) ratio was found to have profound effect on the core content, encapsulation efficiency, capsule stability.
The core content was found to increase with the increasing CM ratio while the encapsulation efficiency continuously decreased with increasing CM ratio. The core content reached a maximum at CM ratio of-3.7. Also the encapsulation efficiency was
found to have a strong dependence on PC ratio and a maximum encapsulation efficiency of 97%, along with the core content of 70% was obtained with CM ratio of-3.7 along with the PC ratio of 6. The microcapsules were well shaped, i.e. round and regular, with narrow size distribution at these conditions. The PCM microcapsules were found to be stable to heat treatment at 150°C for 8 hrs.
From the initial experiments it was observed that the emulsification stirring speed of more than 2000 rpm was necessary for forming a good emulsion. It .was found that the stirring speed should be reduced to allow formation of continuous wall, however, the speed should not be reduced immediately after adding the monomer DETA to prevent capsule agglomeration.
Thus it is obvious that the stirring speed & the manner in which monomer is added influences the characteristics of the PCM capsule particularly shell wall properties.Though a small amount of cyclohexane as solvent (PC value 12) was found to help the microencapsulation process, the microcapsules formed in the absence of cyclohexane also showed high core content with high encapsulation efficiency.The microcapsules produced with the core to monomer ratio of-3.7, PC value of 6 gave
the best microcapsules with core content of 70%, and encapsulation efficiency of 97%.
The highest core content 71% was obtained with PC ratio of 12 at core to monomer ratio
The microcapsules were found to be stable to heat at 150 °C for 8 hrs and hot water
INSITU POLYMERIZATION USING MELAMINE FORMALDEHYDE
The microcapsules containing n-octadecane as the core material and melamine-formaldehyde resin as the wall material were synthesized by in-situ polymerization method. The core content and stability of the microcapsules has been improved by preparing reaction emulsion mixture with increase in the formaldehyde to melamine ratio and core to wall ratio from 2.5 to 8 and from 1 to 4, respectively, and by adjusting curing conditions during the encapsulation process. The microcapsules so obtained had a very high core content up to 70% with the heat storage capacity of > 160 J/g. The capsules
were stable at temperatures in excess of 80 °C and also stable to solvent washes. A thermoregulated fabric also prepared, by coating the capsules between the cellulose-polyester sandwiched fabric and with the resultant heat storage capacity of > 100 J/g of the composite.
The preparation of PCM capsule adopting interfacial polymerization is illustrated below:
Toluene-2, 4-diisicyanate (TDI) (3 g) was taken in a conical flask. 5ml of cyclohexane (as a solvent) was added to TDI. The TDI-cyclohexane mixture was stirred at 500 rpm on a magnetic stirrer plate. After stirring the mixture for 5min, 15ml of n-octadecane was added to the TDI-cyclohexane mixture and the total mixture was stirred further at 1000 rpm for 5 min on the magnetic stirrer. The above mixture (oil) was added to a 150 ml of aqueous solution of 0.5% NP-9.5 as surfactant in a 250 ml beaker. The oil in water emulsion was formed by stirring the mixture at 2500 rpm using a high shear mechanical stirrer consisting of a mesh plate around blades. The emulsion was stirred for 5min at the room temperature. The dibutyl tin dilaurate was added to the emulsion while stirring was continued. 1.5 g of diethylene triamine (DETA) dissolved in 20 ml of water was added slowly to the above emulsion while being stirred to initiate the interfacial polymerization. The microencapsulation was carried out at 2500 rpm for 5min at the room temperature. Then the microencapsulation speed was reduced to 1800 rpm. Afterwards the temperature of the system was increased to 60 °C and the microencapsulation was continued for additional 45 min. Thereafter, second dose of 1.5 g of DETA dissolved in 20 ml of water was added to the above reaction mixture to increase the concentration of DETA. This helps in further diffusion of the monomer DETA through the capsule wall for continued polymerization. The microcapsules were filtered out after carrying out the microencapsulation for further 45 min at the same conditions. The filtered capsules were washed in distilled water at 60°C to remove the unreacted DETA and other chemicals. The washed capsules were again filtered out and dried at 40°C for 12 h in air oven.
Illustration of PCM capsules employing in situ polymerization-Preparation of reaction emulsion mixture
Three different approaches were used for preparing melamine-formaldehyde pre-polymer and emulsion of n-octadecane. These are given as below:
1. Addition of pre-polymer to the prepared PCM emulsion
In this method first PCM emulsion was prepared. The distilled water of 200 ml was taken in a beaker, and to this the fine powder of Sodium lauryl Sulphate (SLS) was added as an emulsifler and mixed well. To this solution, n-octadecane was added slowly over 30 minutes while stirring the mixture (the system temperature was maintained at 40 °C to avoid the solidification of the PCM) vigorously using a high shear mechanical stirrer at 3000 rpm. PVA (water soluble form) as a protective colloid was added to the mixture and the stirring was maintained for an additional 30 minutes. The melamine-formaldehyde pre-polymer was prepared separately by adding calculated quantity of formaldehyde and melamine (Table 1) in a beaker. Before heating the solution, the pH of the mixture was brought to 8.5-9.0 using a solution of 10% sodium carbonate. The temperature was raised to 70 °C while continuously stirring the mixture using a magnetic stirrer. The mixture becomes transparent, indicating the formation of MF pre-polymer. The MF pre-polymer, thus obtained, was added slowly into the prepared PCM emulsion and used for further process of encapsulation.
2. Addition of PCM emulsion to the prepared pre-polymer
In this method of preparation of reaction emulsion mixture, the Melamine-formaldehyde pre-polymer and PCM emulsions were prepared separately as described above. However, in this case, the PCM emulsion, thus obtained was added slowly into the prepared MF pre-polymer and used for further process of encapsulation.
3. Preparation of emulsion in pre-polymer
Melamine-formaldehyde pre-polymer was prepared as described above by adding calculated quantity of formaldehyde and melamine (Table 1) in 200 ml of distilled water taken in a beaker. The pH of the mixture was brought to 8.5-9.0 using a solution of 10% sodium carbonate. The temperature was raised to 70 °C while continuously stirring the mixture using a magnetic stirrer. As the mixture becomes transparent, indicative of the formation of MF pre-polymer, the temperature was brought down to 40 °C. To this, SLS was added as an emulsifler and stirred well. Thereafter, n-octadecane was added slowly over 30 minutes while stirring the mixture vigorously using a high shear mechanical stirrer at 3000 rpm. PVA (water soluble form) as a protective colloid was added to the mixture and the stirring was maintained for an additional 30 minutes to obtain reaction emulsion mixture.(Table-removed)Microencapsulation process
To facilitate encapsulation, the high shear mechanical stirrer from the reaction emulsion mixture was replaced with a low shear magnetic stirrer. The system pH was slowly brought down to 3.0 using 10% solution of sulfuric acid while the temperature was raised slowly to 70 °C. These conditions were maintained for an additional two hours. Finally, the capsules were cooled down to room temperature, filtered, washed with distilled water at room temperature and dried at 40 °C in an air oven for 15 hours, and at 100 °C for 90 minutes.
CHARACTERIZATION OF CAPSULES Core content
The core content of the PCM microcapsules was determined using a Perkin Elmer Differential Scanning Calorimeter (DSC) model DSC 7 with intra cooler. The heating
and cooling scans were carried out at ±10 °C per minute between 0 °C to 60 °C. The core content was calculated as per the following equation.
Heat effusion of the microcapsules containing
Core content (%) = PCM in J/g x 100
Heat of fusion of pure PCM in J/g
Microcapsule stability to solvent wash
This apparently represents the percentage of perfectly formed microcapsules i.e. capsules without micropores. This was determined by calculating the ratio of the core content of the solvent washed microcapsules to that of the distilled water washed microcapsules expressed as a percentage.
For the solvent wash, 0.5 g of dried microcapsules was washed with the 15 g of cyclohexane (solvent for n-octadecane) for 10 minutes at the room temperature.
Core content of the solvent washed
Microcapsule stability to microcapsules x 100
solvent wash (%) = Core content of the distilled water washed
Encapsulation efficiency (or yield)
This is determined as the total amount of core content present in all microcapsules taken as a percentage of the amount of the core material taken during encapsulation process.
Scanning electron microscope (SEM) of make Cambridge Instruments with the model of Stereo scan 360 and optical microscope by Leica model TK-C1380E were used to study surface morphology and size distribution. A drop of microcapsule dispersion was placed on sample stub, dried and coated under vacuum with silver for SEM studies. The size distribution of the microcapsules was determined by measuring diameters of 250 microcapsules under optical microscope.
The mechanism behind the melamine-formaldehyde resin formation involves initially (step I), the reaction between melamine and formaldehyde at a temperature of 65-70 °C leads to the formation of methylolmelamines. Depending upon the relative amounts of melamine and formaldehyde used, the initial products can be di-, tri-, tetra-, penta-, or hexamethylolmelamines. On further heating, condensation of melhylolmelamines takes place with liberation of water molecules and formation of ether linkages (step II a). On subsequent reaction this ether linkages may further converts to form methylene bridges with the liberation of formaldehyde (step II b). Further heating of pre-polymer in the presence of hydrogen ions turns the low molecular weight resin into a crosslinked structure as shown in (III). A fully cured and high molecular weight MF resins shows excellent resistant towards organic solvents.
(Figure removed) the strength of wall material is important in case of encapsulation of PCM and thus influences the thermal and solvent stability. In the preparation of pre-polymer hexamehylolmelamine is known to form in the presence of excess formaldehyde, i.e. more than six moles (preferably eight moles) of formaldehyde for one mole of melamine on heating. In this particular case the number of available reactive groups to react with the formaldehyde is low that results in slow condensation, when compared to the low formaldehyde and melamine molar ratios. The result of heating leads to further condensation of hexamethylolmelamine that eventually yields to form a high molecular weight melamine-formaldehyde resin. It is the most stable and had a very less water solubility than the other methylolmelamines.
The strength of the wall material was influenced by controlling the encapsulation process temperature and pH profile. Thus, ultimately controls the when the condensation of MF pre-polymer occurs and entrap the n-octadecane oil droplets. The encapsulation was initiated at 40 °C by slowly lowering the pH in steps, and that result in the slow condensation of MF pre-polymer. The polymerized products slowly deposit over the oil droplets. The temperature increased to 45 °C meanwhile the pH brought down 4 and kept there for sufficient time to allow the polymer particle to properly encapsulate the n-octadecane. To initiate the full condensation of all the available species of melamine and formaldehyde, the temperature increased to 70 °C and pH brought to 3, thus gives the good protection to oil droplets.
Alternately, the capsule wall is strengthened by controlling the temperature and time profile of curing. Curing results in the reaction of amine and methylol groups that are still available in the product. This reaction split off formaldehyde and forms strong methylene bridge, thus the fully cured and high molecular weight MF resin is achieved. This curing is effective at 160 °C for 20 minutes for normal MF resin product, but the curing of PCM encapsulated MF resin capsules it is preferred at 100 °C for l'/2 hrs and these, conditions prevents the leakage of n-octadecane . In our case the curing conditions has been modified to slowly release the condensation by product formaldehyde. The capsules were dried at 40 °C for 15 hrs for slow condensation and the temperature was raised to 100 °C and kept there for 1 !/2 hrs for full condensation.
The formaldehyde to melamine (fm) ratio affects capsule loading amount, encapsulation efficiency and solvent stability. Figure 3 (accompanying this specification) illustrates clearly that the core content increases from 40.8 to 70.2 with the increase in fm ratio from 2.5 to 8 respectively. This can be ascribed that the increase in fm ratio there is increasing amount of formaldehyde to that of melamine used, while the total amount of monomers being same i.e. with constant core to wall ratio of 2. During the condensation of methlolmelamine, formaldehyde evaporates to form methylene bridge that results in low amount of wall material is required to coat the oil particles. It is worth while to mention that the encapsulation efficiency also increases as the fm molar ratio increases. It is the result of increasing the core content, thus increase the total amount of oil particles being encapsulated.
There was no stability to solvent wash for the samples fm2.5, fm3.5 and fm4.5. This indicates the wall material formed over the PCM is not satisfactorily crosslinked. In these samples the majority of the species formed were trimethylolmelamines in MF pre-polymer process, and it is less stable than hexamethylolmelamines thus further condensation leads to poor wall structure. The poor wall structure again supplemented that at low fm molar ratios, the available reactive site in melamine is comparatively high, thus leads to faster condensation. The samples produced at fm6 and fm8 forms the hexamethylolmelamines in the preparation of pre-polymer. Further condensation on heating leads to slow release of formaldehyde that results in better crosslinked structure with high molecular weight MF resin. Therefore, capsules obtained with the high fm molar ratio exhibited good stability to solvent wash.
Effect of core material on core content, efficiency and solvent stability
The results on the effect of core material showed that there was a gradual increase in core content up to the core to wall ratio of 3.5 and then started decreases (Fig.4 accompanying this specification). The point at which the core content started decreasing can be explained that at the high amount of core material, there is no sufficient wall material to properly encapsulate the oil particles. The effect core material on encapsulation efficiency can be explained that initially the efficiency stood at high value then started decreases as the core material increases, but there was no significant variation in the encapsulation efficiency. The effect of stability of the capsules to solvent showed almost a decreasing trend with increase in core material and reaches to a minimum value of 59.2%. This can be described that with the increase in the core material the surface area of the oil particles to encapsulate increases with constant amount of wall material i.e. constant formaldehyde to melamine molar ratio.From the above disclosure, it is inferred that the suitable ratio of core to wall material lies between 2 and 2.5 with the formaldehyde to melamine molar ratio 6-8 to achieve capsules with high core content, efficiency and thermal and solvent stability.Surface morphology and Size distributionThe micrograph of the capsules obtained in optical microscope showed that capsules were found to be almost circular with a perfect periphery. It was found in SEM that the capsules were fused together this may be due to the aggregation of the capsules at high curing temperature of 100 °C (Fig.l). The size distribution of the microcapsule showed that capsules are in a narrow range of distribution between 1.Sum and 6um with the average size of about 3.5 microns obtained (Fig.2).
Thermoregulation of the coated fabricFabrication of thermo regulated fabric using capsules of present invention The coating paste was prepared by blending, binder, RAN 5000 as thickener, and di-ammonium hydrogen phosphate as curing agent (or catalyst) in distilled water. To this, the microcapsules were added and mixed well to get uniform mixture. The ratio of microcapsules and the binder paste was kept at 1:1.4. The mixture was coated on the
cotton, polyester, or nylon fabric using a doctor blade, the coating was covered with another fabric and sandwiched configuration was cured at 120 °C for 20 minutes. The thermoregulated fabric showed the heat storage capacity of 105 J/g in heating cycle at 32.7 °C, and the same energy was released back in cooling cycle at 25.0 °C at the scanning rate of ±3 °C .
Application of PCM capsules:
1 Managing the thermal barrier properties of garments, in particular for use in
2 Outdoor wear thermoregulating garments (parkas, vests, thermals, snowsuits and
3 Extreme winter clothing materials such as being designed to combat cold, textiles
containing PCMs also helps to combat overheating, so overall the effect can be
described as thermoregulation.
4 Energy conservation applications such as in maintaining the indoor temperature
5 Used in automobiles seating or roofs and side panels to regulate the temperature.
6 To preserve the eatables by providing insulation (such as thermal storage
7 Often, active cooling of such electronic components, particularly delicate TR
modules, Impatt diodes, data recorders, containers for chemicals and munitions,
batteries and the like, is not feasible; and even when it is feasible, it requires
continuous high energy cooling. Encapsulated PCM can be incorporated in the
heat sinks used for cooling the systems.
8 Cooling or warming body wrap in the emergency treatment of strokes, heart
attacks, or other injuries requiring reducing or increasing the body temperature
1. A Phase Change Material (PCM) Capsule comprising of core essentially
consisting of hydrophobic aliphatic alkanes with 8 to 25 carbon and wall
consisting of polymeric material selected from urea formaldehyde, melamine
formaldehyde resins, characterized in that the capsule has 'at least 50% core
content, maximum core to wall ratio of 4, formaldehyde to melamine F/M ratio of
more than 4, heat storage capacity of 160 J/g or more, particle size distribution of
less than 10 microns, and heat, temperature, & solvent stability such as herein
2. A PCM capsule as claimed in claim 1 wherein the hydrophobic aliphatic alkanes
constituting core are n-octadecane or other alkanes, alkyl alcohol or organic acids
such as Dodecane, Tridecane, Tetradecane,Paraffin, Pentadecane, Hexadecane,
Heptadecane, 1-Dodecanol, Octadecane, Nonadecane, Eicosane, 1-Tetradecanol,
Heneicosane, n-Octacosane, n-Heptacosane, n-Hexacosane, n-Pentocosane, n-
Tetracosane, n-Tricosane, n-Docosane , n-Homeiocosane, or any other organic
molecules immiscible with water and inert to encapsulating compounds,
3. A PCM capsule as claimed in claim 1 wherein the particle size distribution has
particle size of less than 1 p. preferably more than 100 nanometers and less than
10 n, further the mean particle size preferably is of 1-4 microns with stirring
speed of 2500 and between 100 nanometer to 1 u, with stirring speed of 2500 to
30000 during emulsification.
4. A PCM capsule as claimed in claim 1 wherein the core to wall ratio in the range
of 1 to 3.5 preferably 1.8 to 2.5.
5. A PCM capsule as claimed in claim 1 wherein the F/M ratio is more than 6
preferably 6 to 8, more preferably 8.
6. A PCM capsule as claimed in claim 1 wherein the wall porosity is of submicron
7. An insitu polymerization process for preparation of PCM capsule as claimed in
claim 1 comprising contacting hydrophobic aliphatic alkanes with 8 to 25 Carbon
atoms and melamine formaldehyde pre-polymer in a manner such as herein
described to prepare emulsion and subject to regulated temperature at a pH below
4 to develop capsules followed by isolating, drying and curing the capsules.
8. A process as claimed in claim 7 wherein the contacting is performed by (a)
adding MF pre-polymer emulsion to PCM emulsion, (b) adding PCM emulsion to
MF pre-polymer emulsion, or (c) mixing predetermined quantity of aqueous
solution of FM pre-polymer and PCM solution in presence of anionic or nonionic
surfactant, protective colloid under vigorous stirring.
9. A process as claimed in claim 8 wherein the emulsion of MF pre-polymer used
for addition to PCM emulsion was maintained at pH of greater than 7, preferably
between 8.5 to 9.0 at in molten form and the addition was conducted slowly & at
more than 2000 rpm.
10. A process as claimed in claim 7 wherein the emulsion pH was maintained at or
below 4.0 preferably between 2.5 to 3.0 and temperature at 60 to 90°C, preferably
70°C till completion of capsule formation.
11. A process as claimed in claim 7 wherein the drying is effected at a minimum
temperature of 40°C and curing is carried out at 100-130°C, preferably at 100°C
for about 1 to Ihr and thirty minutes.
12. A process as claimed in claim 7 wherein the anionic surfactant used is sodium
lauryl sulphate and protective colloid used is PVA.
13. A Phase Change Material (PCM) Capsule substantially as herein described with
reference to the drawings.
14. An insitu polymerization process for preparation of PCM capsule substantially as
herein described with reference to the drawings.
|Indian Patent Application Number
|PG Journal Number
|Date of Filing
|Name of Patentee
|INDIAN INSTITUTE OF TECHNOLOGY, DELHI
|HAUS KHAS, NEW DELHI-110016
|PCT International Classification Number
|PCT International Application Number
|PCT International Filing date