Synthesis of Microcapsules with Beeswax Core and Phenol-Formaldehyde Shell by Taguchi Method †

: The Taguchi method was used to evaluate effect of process parameters in microencapsulation process of beeswax with resorcinol modified phenol-formaldehyde shell. Orthogonal array of 5 3 was constructed to study effect of process parameters core to shell ratio, surfactant concentration and agitation speed on control parameter core content. The amount of core content is directly proportional to heat storing capacity of microcapsules. Surfactant concentration, core to shell ratio, and agitation speed were optimized at 3%, 1:1, and 800 rpm, respectively. The microcapsules synthesized with optimized process parameters values possessed spherical morphology and heat transition enthalpy 148.93 J/g within temperature range 35 – 62 °C.


Introduction
Beeswax is naturally available phase change material (PCM) with peak phase transition enthalpy in temperature range 60-68 °C and can be used in variety of thermal energy storage applications.Beeswax studied for its service years and thermal stability [1].Beeswax can work for 37 years if there were three transitions per day.Beeswax is thermally stable up to 250 °C.It is suitable for using in poultry incubator.Beeswax incorporated in cement and thermal diffusion of composite tested [2].The beeswax incorporated had melting enthalpy of 242.81 J/g at 62.95 °C.Composites was sandwiched between hot plate and cold-water circulating assembly.Beeswax used as coarse aggregate and rice husk used as fine aggregate in concrete mixture [3].Increase in incorporation percent of rice husk reduced compressive strength of concrete.For 5%, 10% and 15% rice husk incorporation, compressive strength was 10.28 MPa, 6.71 MPa and 4.33 MPa.Thermal conductivity of composite obtained as 0.022 W/m°C and thermal diffusivity of composite was calculated as 2.89 × 10 −6 m 2 /s.Beeswax used in parabolic solar concentrator [4].It reduced fluctuation in efficiency of concentrator at the time of low and high solar radiation.Beeswax melt blended with copper oxide, zinc oxide and soot particles [5].Nanoparticle incorporation increases thermal conductivity but reduces latent heat capacity and transition temperature.Soot particle/ beeswax composite showed highest conductivity of 2.89 W/mK.Modified CNT (5%) incorporated in beeswax by vacuum impregnation [6].Incorporation of CNTs increased thermal conductivity.The composite had reduced melting enthalpy of 115.5 J/g at 60 °C.Beeswax was encapsulated in AgBr shell by precipitation of AgBr at beeswax micelle wall [7].Microencapsulated PCM (MPCM) was thermally stable up to 220 °C.The obtained beeswax microcapsules had highest melting enthalpy of 99 J/g at 58 °C.Taguchi is method of determining effect of various process parameters on control parameters.It helps in obtaining correlation between variance of input parameters on control parameters with lesser formulations.Microencapsulation is most widely used technique for form-stabilizing PCM with multiple process parameters.The studied literature for beeswax form-stabilization in thermal energy storing applications have not evaluated process parameters with Taguchi method though it can contribute to deep understanding of process.In this study, effects of process parameters are studied in microencapsulation of beeswax with phenol-formaldehyde shell with Taguchi method for determining optimized values of process parameters.

Methods
SD Fine chemicals Pvt. Ltd. supplied phenol and formaldehyde.SRL Mumbai supplied the beeswax.Loba chemie supplied the polyvinyl alcohol (PVA).Resorcinol, xylene, and ammonium chloride were obtained from Research Lab Pvt. Ltd.DI water was utilized throughout experiments.
Microcapsules prepared by suspension polymerization with PVA surfactant.2.1 g phenol and 0.5 g ammonium chloride were dissolved in PVA-water solution for 30 min and adjusted to pH 7. Beeswax-xylene solution ultrasonicated at 60 °C for 5 min.This solution was added to 3.35 g of formaldehyde solution and heated to 65 °C under stirring at 500 rpm for 2 h.At pH 3, 0.5 g of resorcinol was added.Reaction was continued for 2.5 h.Obtained microcapsules were vacuum filtered, washed & dried.Core content was obtained by determining change in weight percentage of crushed microcapsules dissolved in xylene and dried microcapsules.Core content of microcapsules was calculated in triplets for different levels of parameters and average value was taken for calculation which is mentioned in Table 1.Larger the better approach was used to construct 5 3 orthogonal array.Microcapsules morphology was studied with optical microscope (Olympus BX41) and scanning electron microscope (SEM).Thermal energy storage characteristics were determined with differential scanning calorimeter (Shimadzu DSC-60).

Results and Discussion
Microencapsulation is a process of engulfing core material with shell material.In the studied polymeric micrroencapsulation process, dispersion of microscaled oleophobic beeswax grains were coated with resorcinol modified phenol-formaldehyde polymer with in-situ polymerization process which is depicted in Figure 1.Higher reactivity of resorcinol with formaldehyde help in faster shell formation.The amount of core content is directly proportional to heat storing capacity of microcapsules.The factors that must be optimized include the core to shell ratio, surfactant concentration, and mixing speed.This will be accomplished using the Taguchi technique.Thus, Taguchi array was constructed with three process parameters and one control parameter.Three factors were changed at five different levels.The core-to-shell ratios were changed as follows: 0.5:1, 1:1, 1.5:1, 2:1, and 2.5:1.Surfactant concentrations were between 1%, 2%, 4%, and 5%.The agitation speed was changed between 400, 600, 800, 1000, and 1200 rpm.Table 1 shows the batches that were investigated.Signal to noise (SN) ratio was calculated with larger the better option in minitab software in order to maximize control parameter.Changing all of the parameters necessitates the production of 5 3 (125) batches.The optimal parameter values, however, can be achieved in only 25 batches using the Taguchi orthogonal array.
The change in values of process parameters on SN ratio can be understood with Figure 2. The horizontal axis represents values of each process parameters tested in formulations and vertical axis represents SN ratio.SN ratio showed steep increase with increase in surfactant concentration up to 3 wt% followed by decrease.Increasing the concentration of surfactant above 3 wt% level resulted in a finer emulsion with lower core content.In the case of core to shell ratio, 1:1 and 0.5:1 ratios in the graph resulted increase in core content and thus increase in SN ratio.Reduction in SN ratio above 0.5:1 level is from decrease in shell thickness of microcapsules.A ruptured thin shell may have a low core content.The figure depicted speed to SN ratio correlation in last section.Increase in speed up to 800 rpm aids in the development of core/shell morphology; however, beyond this speed may the shell rupture.This relation can be clearly observed with SN values.Considering effect of SN ratio to maximize core content, the optimum values for surfactant concentration, core to shell ratio, and agitation speed were selected as 3%, 1:1, and 800 rpm respectively.

Figure 2. Main effects plots of SN ratio.
A batch was synthesized with optimum parameter values and characterized for its thermophysical properties.In image J application, the size of the MPCM with optimum process parameter values was determined by taking the average of 100 measurements.The average size was 62.61 µ m. Figure 3a depicts an optical micrograph of microcapsules. Figure 3b is SEM image of single microsphere with diameter of 70.33 µ m.With image analysis, successful microcapsule formation can be postulated.The microcapsules that were created were spherical in form.The reaction conditions proposed yield tiny sized microcapsules that may be easily utilized in coating applications with thinner coatings.In the temperature range of 35-62 °C, the melting enthalpy of MPCM was 148.93 J/g. Figure 4 depicts the melting thermogram.The temperature range found for the phase transition temperature may be utilized to store various food items such as hot served cuisines, building materials, and solar energy storage materials.The two peaks of the phase transition allow heat to be stored across a wider temperature range.

Conclusions
The impact of surfactant concentration, core to shell ratio, and agitation speed on MPCM core content was investigated.The optimal values for surfactant concentration, core to shell ratio, and agitation speed were 3%, 1:1, and 800 rpm, respectively.MPCM's spherical shape and micrometer size allow it to be used for a wide range of applications.The melting enthalpy and temperature range of the phase transition are appropriate for various thermal energy storage applications.