Effect of curing temperature on the performance of microencapsulated low melting point paraffin using urea-formaldehyde resin as a shell

Abstract

Microencapsulated phase change materials (MicroPCMs) containing paraffin with low melting temperature were prepared by in situ polymerization using urea-formaldehyde resin as shell material. The curing stage was very crucial in determining the properties of the MicroPCMs. The effects of curing temperature on thermal properties, diameter distributions and surface morphology were systematically investigated by using differential scanning calorimetry, thermogravimetry analysis, a mean particle size distribution meter and scanning electronic microscopy. The obtained results indicate that the optimum curing temperature is 65℃. Under this condition, the paraffin mass content of MicroPCMs is 52.8%, and the enthalpy of MicroPCMs containing paraffin of 52.8% is 74.2 J/g. In addition, the mean volume particle diameter of MicroPCMs is 4.57 µm with particle size distribution of 0.37. The thermal resistant temperature of MicroPCMs is 220℃. Based on the results, it can be considered that MicroPCMs prepared at a curing temperature of 65℃ have good energy storage potential and temperature resistance. Therefore, they can be used to produce nonwoven materials by melt-blow.

Keywords

Microencapsulation urea-formaldehyde resin in situ polymerization curing temperature

Unlike conventional (sensible) materials, when phase change materials (PCMs) begin their phase transition within a defined temperature range, they can absorb, store and release large amount of latent heat with very minor variation. Hence, PCMs have been widely used in the field of thermal control and thermal energy conservation.¹ Paraffin is the best chosen PCM due to its characteristics such as low cost, stable chemical properties and high storage energy capacity.^2,3 However, it is necessary to encapsulate PCMs using appropriate materials as shells in order to prevent melt from flowing during solid–liquids phase transition. The diameters of the microcapsules are usually in the range of 1–1000 µm. The microencapsulation of PCMs not only improves stability, resistant to corrosion and utilization of PCMs, but also strengthens heat transfer performance. Therefore, it has an extensive potential application prospect in some fields of energy-saving building materials,^4,5 solar energy storage utilization,^6,7 thermo-regulated fibers^8–11 and foams.¹²

There are several methods of producing Microencapsulated phase change materials (MicroPCMs), such as spray-drying,¹³ in situ polymerization,^14,15 interfacial polymerization^16,17 and so on. Many experiments have indicated that in situ polymerization method is one of the most promising techniques to encapsulate liquid PCMs from emulsions.^18–20 Jin et al.¹⁸ prepared the capsules containing paraffin as a phase change core, and polymerization of urea-formaldehyde (UF) prepolymer onto the core using the in situ method in the presence of hydrolyzed styrenemaleic anhydride copolymer as emulsifier. The particle size and phase change behavior were studied.

UF resin as shell materials has good toughness and strength, and also excellent barrier properties. Therefore, many papers on MicroPCM preparation using UF resin as shell materials by in situ polymerization have been published.^14,18,19

However, few papers reported systematically the effect of curing temperature on the overall performance of MicroPCMs. In the subsequent studies, modified Polypropylene (PP) melt-blow nonwoven will be prepared by blend spinning with high melt index PP and UF/low melting point paraffin MicroPCMs. Therefore, it does require the use of high-performance–price ratio paraffin as core materials and MicroPCMs with high temperature resistance and thermal storage capacity. Since curing temperature directly affects the encapsulated ratio and the structure of UF resin, the purpose of the present study is to mainly investigate the effects of curing temperature on morphology, particle size and distribution, thermal storage capacity and temperature resistance of MicroPCMs under the condition that other preparation conditions are relatively fixed.

Experimental materials

Paraffin (mixture of various n-alkanes, melting point is 29.4℃, industrial grade) was purchased from Nanyang Wax Fine Chemical Plant as core material. Poly(styrene-co-maleic anhydride) (SMA, industrial grade) with an average molecular weight of 5500 was purchased from Sigma-Aldrich. Polyoxyethylene octylphenol ether (OP-10, >96%) was purchased from Tianjin Kermel Chemical Reagent Co., Ltd. Urea (>99%) and formaldehyde (37 wt.% aqueous solution) used as shell materials were purchased from Luoyang Chemical Reagent, China. Acetic acid (>99.5%), sodium hydroxide (>96%), triethanolamine (>85%) and ammonium chloride (>99.5%) were purchased from Kaitong, Tianjin Chemical Reagent Co., Ltd.

Prepolymer solution synthesis

Urea (6.0 g), formaldehyde (14.5 mL) and deionized water (40 mL) in a three-mouth flask were stirred and dissolved. Then the pH value of the reaction system was adjusted to 8–9 with triethanolamine solution (65 wt.%). Next, the reaction was at 60℃ for 60 min with a stirring rate of 200 rpm. Finally, a transparent prepolymer solution can be obtained.

Emulsion preparation

Paraffin (15.0 g), 1.35 g SMA/OP-10 composite emulsifier (mass ratio of SMA and OP-10 as 4:1) and 80.0 mL deionized water were emulsified mechanically with a stirring rate of 8000 rpm for 20 min. Then the low melting paraffin emulsion was obtained.

MicroPCMs fabrication

The prepolymer solution was dropped into the emulsion slowly. After all of the prepolymer was added, the pH value of the solution was adjusted to 4.5 with acetic acid solution (65 wt.%) with a stirring rate of 200 rpm at 40℃ for 60 min. Then 1.0 mL ammonium chloride solution (10 wt.%) providing hydrogen ions to the reaction system was added into the solution to ensure a smooth curing reaction. In the curing process, the curing temperatures were 40℃, 50℃, 60℃, 65℃ and 70℃, respectively (as shown in Table 1). Next, the pH value of the solution was adjusted to 8.0 with 10 wt.% sodium hydroxide solution. The obtained MicroPCMs were filtered and washed with 30 wt.% ethanol–water solution. Finally, the wet cake was dried in a vacuum oven.

Table 1.

Reaction conditions

	Sample no.
	1	2	3	4	5
Curing temperatures (^°C)	40	50	60	65	70
Paraffin (g)	15.0
SMA/OP-10 (g)	1.35
Mass ratio (SMA:OP-10)	4:1
Formaldehyde (37 wt% aqueous solution) (mL)	14.5
Urea (g)	6.0
Triethanolamine solution (mL)	0.5
Acetic acid solution (mL)	5.0
Ammonium chloride (mL)	1.0

Reaction mechanism for urea and formaldehyde

During the preparation of prepolymer, the nucleophilic addition reaction of urea and formaldehyde will form monomethylol urea and dimethylol urea under alkaline conditions, according to Figure 1. In the experiment, the molar ratio of formaldehyde and urea was 1.8–2.0:1 in order to form enough dimethylol urea and ensure a certain crosslinking degree for UF resin at the polycondensation stage.

Figure 1.

Methylolated reaction scheme of urea and formaldehyde under alkaline conditions.

The polymerization process includes two stages, acidification and curing. Under the weak acidic condition, the hydroxymethyl group in the prepolymer reacts with the imino group on the other prepolymer and the hydroxymethyl group on the other prepolymer to form a linear or branched polymer. The main reactions are shown in Figure 2.

Figure 2.

Reaction scheme in the acidification stage.

After the prepolymer is acidated, the curing and crosslinking reactions start by adding ammonium chloride solution into the system. The structure of UF resin is shown in Figure 3. The curing temperature determines the final molecular and crosslinking degree of UF resin, which will influence the properties of MicroPCMs.

Figure 3.

Urea-formaldehyde schemes at the curing stage.

Characterization

Structure

The structural analyses of paraffin, UF resin and MicroPCMs were performed using a Fourier transform infrared (FTIR) instrument (Nicolet Protégé 460 spectrometer, resolution is 0.4 cm⁻¹ and the wavenumbers range is 4000-400 cm⁻¹) on a KBr disk.

Morphology

The morphology of the microcapsules was observed using scanning electron microscopy (SEM) (Hitachi S-3000N, Japan). The microcapsules were dispersed at a low concentration in ethanol solution using the ultrasonic method. A drop of MicroPCMs/ethanol dispersion was dropped on a stainless steel SEM stub and allowed to air-dry overnight. Prior the test, the samples were sprayed gold.

Thermal property

The thermal properties of MicroPCMs were measured using differential scanning calorimetry (DSC, NETZSCH 204 F1) at a heating or cooling rate of 10℃ min⁻¹ in the range of 0–100℃ under a nitrogen atmosphere. The melting enthalpy of paraffin is constant in the measured temperature range; here it is 140.5 J/g (DSC melting enthalpy). The content of paraffin in the MicroPCMs can be estimated according to the measured enthalpy (Equation (1)):²¹

Paraffincontent = \frac{| Δ H_{MicroPCMs} |}{| Δ H_{paraffin} |} \times 100 %

(1)

where

Δ H_{MicroPCMs}

and

Δ H_{paraffin}

are the melting enthalpy (J/g) of MicroPCMs and paraffin, respectively.

Particle size

The diameter distribution of the microcapsules emulsion was carried out on a HYL-1076 laser particle size distribution meter (Hengyu Instrument Co. Ltd, Dandong). The microcapsules emulsion with ultrasonic oscillation treatment for 10 minutes was measured.

Thermal stability

The thermal stability of MicroPCMs was determined with thermogravimetric analysis (NETZSCH 209F1 Iris) from 20℃ to 550℃ at a heating rate of 10℃ min⁻¹ in a nitrogen atmosphere.

Results and discussion

FTIR analysis of the products

The compositions of paraffin and MicroPCMs with different curing temperatures were characterized by FTIR, as shown in Figure 4. The spectra (a), (b) and (c) show a strong absorption band at 2937–2848 cm⁻¹, which associates to the aliphatic C-H stretching vibration of the core material paraffin. Comparing (b) and (c), N-H stretching vibration peaks (at 3373 cm⁻¹) become wider and blunter and move to a lower wavenumber with the increase of curing temperature, which is due to the participation of hydrogen of the amino group in polymerization and the increase of the polymerization degree, which made the characteristic peaks for N-H diffuse because the reaction is unfavorable for N-H stretching vibration. The absorption peak at 1670–1550 cm⁻¹ is assigned to the amide II characteristic peaks from N-H plane bending vibration and partial C-N stretching vibration coupling and mainly from N-H plane bending vibration. Amide II peaks gradually move toward lower wavenumbers from 1670–1591 cm⁻¹ to 1653–1556 cm⁻¹ with the increase of curing temperature; this is due to the participation of hydrogen of the amino group in reaction and this leads to the increase of polymerization degree, which is unfavorable to the stretching vibration of C-N. The absorption peak at 1430–1250 cm⁻¹ is assigned to the amide III characteristic peaks. The peaks become weaker and gradually move toward lower wavenumbers with the increase of curing temperature, suggesting the decrease of -NHCH₂ content and the increase of crosslinking structure. The absorption peak at 1180–1043 cm⁻¹ is also assigned to the amide III characteristic peaks from C-H stretching vibration in -NHCH₂-, indicating the formation of methylene bridge structure.²² The above analysis shows that the reaction degree of UF and crosslinking degree increase.

Figure 4.

Fourier transform infrared spectra of (a) paraffin, (b) sample no. 1 and (c) sample no. 4.

Morphology of MicroPCMs

SEM micrographs of the products prepared with different curing temperatures are shown in Figure 5. When the curing temperature was 40℃, MicroPCMs showed broken and collapsed structures. The morphology of MicroPCMs tended to become regular with the increase of curing temperature. When the curing temperature was 65℃, MicroPCMs were regular spherical, almost without adhesion phenomena, but with a higher encapsulated ratio. When the curing temperature was 70℃, break phenomena and adhesion increased. These results indicate that polymerization degree and the strength of UF are lower when the curing temperature is low, so it is destroyed at the post-processing. The crosslinking degree and encapsulated ratio increases with the increase of curing temperature; this is due to the participation of hydrogen of the amino group in the reaction and the formation of the methylene bridge structure. However, if the curing temperature is too high, the polymerization of UF is strong, which results in the adhesion of Microcapsules and the content of collapsed capsules increases.

Figure 5.

Scanning electron micrographs of Microencapsulated phase change materials prepared with different curing temperatures: (a) sample no. 1; (b) sample no. 2; (c) sample no. 3; (d) sample no. 4; (e) sample no. 5.

Particle size distribution

The effects of curing temperatures on particle size distribution are depicted in Figure 6 and Table 2. The obtained MicroPCMs have a relatively monomodal distribution curve, as shown in Figure 6. Particle size distribution can be used to denote the distribution uniformity: the smaller values, the better distribution. Particle size distribution index expresses the difference degree of large and small particles. The smaller the value, the better the distribution. The volume average particle size and particle size distribution index of MicroPCMs first decrease and then increase with the increase of curing temperature. Among them, the volume average particle diameter is larger for sample no. 1 and 5, and the particle size distribution is wide. However, the volume average particle size (4.57 µm) is smaller and the particle size distribution was more uniform and the particle size distribution index was 0.37.

Figure 6.

Effect of curing temperature on particle size distribution of Microencapsulated phase change materials: (a) sample no. 1; (b) sample no. 2; (c) sample no. 3; (d) sample no. 4; (e) sample no. 5.

Table 2.

Effect of the curing temperatures on mean volume diameter and particle size distribution index of Microencapsulated phase change materials

Sample no.	Mean volume diameter (µm)	Particle size distribution index
1	12.68	0.53
2	6.17	0.42
3	5.57	0.38
4	4.57	0.37
5	11.23	0.44

When the curing temperature is lower, the activity of the -NHCH₂- group is lower, and the resin is easy to stick due to insufficient polymerization degree and paraffin leaking out. The crosslinking degree, intensity and encapsulated ratio of resin can be improved and the diameter distribution is narrower with the increase of curing temperature. However, when the curing temperature is higher, such as 70℃, the polymerization speed of UF is too fast. This resulted in larger volume average particle diameter and wider distribution.

Thermal properties of MicroPCMs

The DSC heating curves of paraffin and MicroPCMs are shown in Figure 7. The melting properties of paraffin and MicroPCMs prepared with different curing temperatures are presented in Table 3. The melting enthalpy of pure paraffin is 140.5 J/g. As shown in Figure 7 and Table 3, the latent heat increased first and then decreased sharply with the curing temperature increasing. The latent heat of MicroPCMs with the curing temperature of 65℃ is higher than other samples; the content of paraffin is 52.8%. The latent heat of MicroPCMs was only 26.5 J/g even if the curing time was extended to 180 min at 40℃. This is due to the lower reaction activity of the amino and imino group, resulting in the crosslinking degree of UF being small, the stability of MicroPCMs being poor and MicroPCMs with a few core materials forming because the paraffin is leaks easily. The latent heat increases with the curing temperature increasing. This is due to the generation of UF equal with the package of paraffin and the strength of UF made greater and it cannot easily be damaged. When the curing temperature is above 70℃, there is not enough time to wrap core materials due to the fast formation speed of UF resin, resulting in lower paraffin content of MicroPCMs. In addition, it can be seen that the peak of the melting point and melting range are different from paraffin. For example, the melting peak temperature of MicroPCMs prepared at curing temperature of 65℃ is 3.3℃ higher than that of paraffin, and the peak becomes wider than that of paraffin, which may be due to the variation of the shell thickness¹⁹ or different crystalline environment.²³

Figure 7.

Differential scanning calorimetry thermograms of paraffin (a) and Microencapsulated phase change materials prepared with different curing temperatures: (b) sample no. 1; (c) sample no. 2; (d) sample no. 3; (e) sample no. 4; (f) sample no. 5.

Table 3.

Phase transition properties of Microencapsulated phase change materials from differential scanning calorimetry (DSC) measurements

Sample no.	T_m (^°C)	Transformation range (^°C)	$Δ H_{m}$ (J/g)	Paraffin content (wt.%)
Paraffin	29.4	23.0–31.8	140.5	/
1	21.9	18.0–28.1	23.6	16.7
2	24.8	20.5–30.7	55.1	39.2
3	25.3	22.9–32.8	58.2	41.4
4	26.1	20.1–31.6	74.2	52.8
5	/	/	1.6	1.1

T_m: peak temperature on DSC heating curve; $Δ H_{m}$ : enthalpy on DSC heating curve.

Thermal stability of MicroPCMs

Thermogravimetry analysis (TG) diagrams and parameters of MicroPCMs synthesized at different curing temperatures are shown in Figure 8 and Table 4, respectively. The paraffin starts to lose weight at about 187℃ and completes at about 300℃. MicroPCMs have an initial mass loss of about 10% from 50℃ to 150℃, which is due to the gasification of water and oligomer,²⁴ so the related data about this are not included in Table 4. It is shown from Figure 8 and Table 4 that the weight losses of MicroPCMs with different curing temperatures have many differences. There is only one weight loss for sample no. 5, which indicates that it contains no paraffin; this is due to the decomposition of wall material. There are two weight-loss steps of sample no. 1, 2, 3 and 4, implying that the UF resin shells can prevent the paraffin from losing weight quickly,^25,26 and weight-loss temperature and weight-loss percent of the first step increased with the curing temperature increase, which is because the wall breaks and paraffin leaks out. The second step is from 300℃ to 450℃, which is caused by the decomposition of wall material. From the TG, it can be known that the weight-loss percent of the first weight-loss step is identical with the paraffin content shown in Table 3. The above discussion suggests that MicroPCMs at a curing temperature of 65℃ have good heat storage capacity and good temperature resistance.

Figure 8.

Thermogravimetry analysis diagrams of paraffin (a), Microencapsulated phase change materials prepared with different curing temperatures: (b) sample no. 1; (c) sample no. 2; (d) sample no. 3; (e) sample no. 4; (f) sample no. 5.

Table 4.

Thermogravimetry analysis results of microcapsules with different curing temperatures

Sample no.	Starting point I (℃)	Weight loss percent of I (℃)	Starting point II (℃)	Weight loss percent of II (℃)
1	185.8	23.9	291.6	71.7
2	218.4	39.6	292.2	48.9
3	220.5	42.3	293.5	46.7
4	223.7	54.5	323.5	31.7
5	287.5	81.1	/	/
Paraffin	187.1	98.5	/	/

Conclusions

MicroPCMs based on paraffin core and UF resin shell were prepared via the in situ polymerization process. In this work, the effect of curing temperature on MicroPCM properties was investigated. It was found from the systematical measurements that the optimum curing temperature was 65℃, the paraffin content of MicroPCMs was 52.8% at a curing temperature of 65℃, and the enthalpy of MicroPCMs was 74.2 J/g. Meanwhile, the mean volume diameter of MicroPCMs was the smallest at about 4.57 µm at the curing temperature of 65℃. Moreover, MicroPCMs were regular spherical almost without adhesion phenomena. UF resin shells can prevent paraffin from losing weight quickly, which led to MicroPCMs’ good thermal stability. The good thermal stability is important in practical applications, especially for melt spinning. It is important for melt spinning that the MicroPCMs have smaller size, higher temperature resistance and heat storage capacity. Therefore, blend spinning with high melt index PP and MicroPCMs can produce the melt-blown nonwoven with thermo-regulated functions.

Footnotes

Funding

This work was supported by the Henan Provincial Key Scientific and Technological Project (Grant No. 102102310358).

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