Synthesis and Study of Microcapsules with Beeswax Core and Phenol-Formaldehyde Shell Using the Taguchi Method ^†

Abstract

Phenol-formaldehyde shelled phase change material microcapsules (MPCMs) were fabricated and their processing parameters were analyzed with the Taguchi method. Core to shell ratio, surfactant concentration and speed of mixing are the parameters that were optimized in five levels. The optimized values for the surfactant concentration, core to shell ratio and agitation speed were 3%, 1:1 and 800 rpm, respectively. The obtained microcapsules were spherical in shape. The melting enthalpy of the MPCMs synthesized with optimized processing parameters was 148.93 J/g in 35–62 °C. The obtained temperature range of phase transition temperature can be used for storing different food articles such as chocolate and hot served foods.

1. Introduction

Beeswax has been used as a phase change material (PCM) in various applications such as building applications and solar energy storage applications owing to its phase change temperature around 60 °C. Beeswax has been used in waterborne coating for preparing hydrophobic thermoregulating coating [1]. The dispersion of beeswax/perfluorinated copolymer (used as an encapsulant)/silica nanoparticle was made with a homogenizer to give a 350-nanometer particle size. The obtained material had a phase transition enthalpy of 84 J/g at 61 °C. The addition of thermally conductive nanomaterials in a beeswax composite improves its heat dissipation characteristics. Beeswax has been mixed with copper, aluminum and graphite nanoparticles to increase its conductivity [2]. The prepared composite filled in 55-millimeter capsules and was placed in a heat storage tank. The graphite/beeswax nanocomposite outperforms other nanocomposites with less time for charging and a high discharging time. Bentonite clay has been integrated with beeswax and mixed with concrete [3]. The heat absorption of the composite with PCM increased by 6.67%, but the compression strength was reduced. Modified carbon nanotubes (CNTs) (5%) have been incorporated in beeswax using vacuum impregnation [4]. The incorporation of CNTs increased the thermal conductivity. The composite had a reduced melting enthalpy of 115.5 J/g at 60 °C. Black beeswax has been incorporated into a prototype roof model made from an MDF sheet and covered with EPS foam [5]. An increase in the temperature at night of 3.6 °C was observed. A simulation study projected an energy saved of 67%. Beeswax has been stored in a container with a PV panel [6]. It reduced heat waste from the panel and increased the voltage generated.

The microencapsulation method is an exhaustively used technique for PCM shape-stabilization. This technique has been used in a broad spectrum of applications such as building, medicine, electronics, food, etc. Polymeric encapsulation is characterized by high toughness and a good heat transfer property due to the large surface area of the capsules. In this paper, phenol-formaldehyde shelled PCM microcapsules are fabricated, and their properties are studied.

2. Materials and Methods

Phenol and formaldehyde were purchased from SD Fine chemicals private limited. Beeswax was procured from SRL, Mumbai. Polyvinyl alcohol (PVA) were bought from Loba Chemie. Resorcinol, ammonium chloride and xylene were purchased from Research Lab private limited. Deionized water (DI) was used in all the experimental work.

Beeswax/phenol-formaldehyde core/shell particles were prepared using a suspension polymerization technique. In DI water, 5 wt.% aqueous solutions of PVA was prepared. The solution was mixed by magnetic stirring at 500 rpm until PVA dissolved. Under agitation, 2.1 g of phenol and 0.5 g of ammonium chloride were dissolved in PVA solution for 30 min. The pH of PVA solution was adjusted to 7–8 using an ammonia solution. Different amounts of PCM were added to 10 mL of xylene in beakers and subjected to agitation for 5 min with a magnetic stirrer at 60 °C. An emulsion was allowed to form by adding PVA solution to PCM solution under ultrasonication for 30 min. To another heated container placed inside the heater, 3.35 g of 37 wt.% aqueous solution of formaldehyde and ultrasonicated solution was added. Solution was slowly added to the container and maintained at 65 °C under stirring at 500 rpm for the next 2 h. Then, 5 wt.% of HCl was added to maintain the pH at about 3–4 and 0.5 g of resorcinol was added. The reaction was continued at the same temperature for the next 2.5 h. Microcapsules were recovered by filtration under vacuum. The microcapsules were rinsed with water, washed with xylene, and dried for 24 h.

An accurately weighed sample was crushed and stirred in xylene for 1 h at 70 °C under magnetic stirring. The microcapsules were rinsed with water, washed with xylene, and dried for 24 h. The core content is the percentage of microcapsule weight difference before and after the treatment. The core content was calculated by taking an average of 3 readings. An Olympus BX41 optical microscope was used to measure the size of a microcapsule. The size of microcapsule was calculated taking mean of 100 readings from Image J software. A differential scanning calorimeter (Shimadzu DSC-60) was used to determine enthalpy and phase transition temperature of microcapsules.

3. Results

Core to shell ratio, surfactant concentration and speed of mixing are the parameters that need to be optimized. This can be achieved thoroughly with the Taguchi method. The three parameters were varied in five levels. The core to shell ratio was varied as 0.5:1, 1:1, 1.5:1, 2:1 and 2.5:1. The surfactant concentration was varied as 1, 2, 3, 4 and 5%. The agitation speed was varied as 400, 600, 800, 1000 and 1200 rpm. The batches studied are shown in

Table 1. Taguchi L₂₅ orthogonal array with signal to noise ratio (SNR).

Run (Nos.)	Surfactant Concentration (g)	Core-to-Shell Ratio (In Moles)	Agitation Speed (rpm)	Core Content (wt.%)				SNR (dB)
				R1	R2	R3	Average
1	1	0.5:1	400	59.4	63	61.2	61.2	35.73503
2	1	01:01	600	77.4	68.4	70.2	72	37.14665
3	1	1.5:1	800	67.5	72	71.1	70.2	36.92674
4	1	02:01	1000	52.2	48.6	50.4	50.4	34.04861
5	1	2.5:1	1200	14.4	27	18	19.8	25.9333
6	2	0.5:1	400	75.6	75.6	79.2	76.8	37.70722
7	2	01:01	600	73.8	77.4	81	77.4	37.77482
8	2	1.5:1	800	68.4	68.4	63	66.6	36.46948
9	2	02:01	1000	52.2	57.6	52.2	54	34.64788
10	2	2.5:1	1200	73.8	77.4	81	77.4	37.77482
11	3	0.5:1	400	66.6	72	82.8	73.8	37.36113
12	3	01:01	600	69	81	84	78	37.84189
13	3	1.5:1	800	57	73.5	72	67.5	36.58608
14	3	02:01	1000	67.5	76.5	76.5	73.5	37.32575
15	3	2.5:1	1200	70.5	67.5	73.5	70.5	36.96378
16	4	0.5:1	400	70.5	72.45	73.5	72.15	37.16473
17	4	01:01	600	79.2	79.2	73.8	77.4	37.77482
18	4	1.5:1	800	62.4	66	58.8	62.4	35.90369
19	4	02:01	1000	61.2	58.8	60	60	35.56303
20	4	2.5:1	1200	66	69.6	73.2	69.6	36.85218
21	5	0.5:1	400	48	45	51	48	33.62482
22	5	01:01	600	57	66	61.5	61.5	35.7775
23	5	1.5:1	800	78	74.4	78	76.8	37.70722
24	5	02:01	1000	64.8	66	60	63.6	36.06914
25	5	2.5:1	1200	60	52.5	67.5	60	35.56303

. Varying all the parameters requires fabrication of 5³ (125) batches. However, with the help of the Taguchi orthogonal array, the optimized parameters can be obtained with only 25 batches. The increase in the core content will increase the thermal energy storage property. Thus, larger-the-better form of analysis was chosen.

The effect of the parameter values on the core content can be studied with the main effects plot for SN ratios, which is shown in Figure 1.

Increasing the surfactant concentration gave a finer emulsion with better dispersion. As the surfactant concentration increases above 3 wt.%, the core content reduces. This is the reason for the decrease in the SNR value. An increase in the core content was observed for core to shell ratios 1:1 and 0.5:1. A further increase in the ratio reduced the shell thickness. The ruptured thin shell can show a low core content. An increase in the speed up to 800 rpm helped in the formation of core/shell morphology; increasing the speed above this may rupture the shell. Therefore, the optimized values for the surfactant concentration, core to shell ratio and agitation speed are 3%, 1:1 and 800 rpm.

The size of MPCM with optimized parameters was calculated taking mean of 100 readings in image J software. Thus, the size obtained was 62.61 µm. The optical micrograph was shown in Figure 2. The obtained microcapsules were spherical in shape. The suggested parameters of reaction give small sized microcapsules which can be easily used in coating applications with smaller thickness coatings.

The melting enthalpy of the MPCMs was 148.93 J/g in the range of 35–62 °C. The melting thermogram can be seen in Figure 3. The obtained temperature range of the phase transition temperature can be used for storing different food articles such as chocolate and hot served foods, building materials and solar energy storing materials. The two peaks of the phase transition allow heat storage for a larger temperature range.

4. Discussion

The effect of the surfactant concentration, core to shell ratio and agitation speed on the core content of MPCMs was studied. The optimized values for the surfactant concentration, core to shell ratio and agitation speed were 3%, 1:1 and 800 rpm. The structure of the MPCM is spherical and the size in micrometers allows it to be used for a myriad of applications. The melting enthalpy and temperature range of the phase transition are suitable for thermal energy storing applications.