Synthesis of poly(methyl methacrylate-co-acrylic acid)/ n -eicosane microcapsules for thermal comfort in textiles

Abstract

Homogeneous distribution by adsorption is one of the key issues for application of microencapsulated materials to textiles. This study focused on production and characterization of poly(methyl methacrylate-co-acrylic acid)/n-eicosane microencapsulated phase change materials (MEPCMs) as textile thermal comfort additives with a functional outer surface. For this reason, methyl methacrylate was copolymerized with acrylic acid at three different ratios. The chemical structure, thermal energy storage properties, and thermal stability of microcapsules were investigated by FT-IR spectroscopy, differential scanning calorimetry, and thermogravimetric analysis techniques, respectively. Microcapsules were found to have a thermal energy storage capacity of 50.9–90.9 J/g in the 31.74–36.30℃ temperature interval and they release between −88.4 and −40.2 J/g in the 33.88–35.59℃ temperature interval. Using a scanning electron microscope and a particle size instrument, the spherical morphology and particle size distribution of were determined for the microcapsules produced. The average particle sizes were 22.53 µm, 21.87 µm, and 11.73 µm for microcapsules with increasing amount of acrylic acid content. The microcapsules were thermally stable up to at least 120℃.

Keywords

phase change materials (PCMs)microcapsules microencapsulated PCMs P(MMA-co-AA)DSC

Phase change materials (PCMs) are functional materials that can absorb, store, and release latent energy isothermally during a phase transition. Many researchers have studied PCMs for miscellaneous applications such as solar energy utilization,¹ smart housing,² thermo-regulated fibers, fabrics, coatings, and foams,^3–10 heat transfer materials,¹¹ and agricultural applications.¹² In the applications, microencapsulated phase change materials (MEPCMs) are more effective used owing to their outstanding properties, including micro-size containment, no liquid handling problems, non-corrosion, heat transfer capability, ease of application, etc.

Organic PCMs, like paraffin and fatty acids, have a wide range of applications since they have desirable properties, such as high latent heat, congruent melting/freezing, little supercooling, low vapor pressure, nontoxicity, low cost, and easy availability.^13,14 For thermal comfort in textiles, paraffin waxes have generally been exploited as favorable PCMs. However, direct utilization of paraffins is subject to some restrictions due to leakage, evaporation, and odor problems. Besides, their thermal conductivity is considerably low.¹⁵ In the past decade, microencapsulation technology has been developed and applied to solve these problems.^15,16

Microencapsulation is a process of enclosing micro-sized PCM particles in a shell structure known as a MEPCM (microencapsulated PCM) or PCM microcapsule. A series of physical and chemical methods have been developed for MEPCM preparation. The most common of these are emulsion polymerization,^17–19 interfacial polymerization,²⁰ coacervation,^5,21,22 and in situ polymerization.^23,24 In general, the shells of microcapsules are formed using various natural or synthetic polymers. The choice of a suitable microcapsule production process is based on the choice of the polymeric shell material. According to a literature survey, natural polymers such as gum arabic, agar-agar, gelatin, and silk fibroin were chosen for the complex coacervation method, whereas some synthetic shells, such as polystyrene, polyurethane, and melamine formaldehyde, were synthesized by emulsion, interfacial, and in-situ polymerization methods.

Application of MEPCMs for thermal comfort improvement in textiles is one of the technical fields for personal heating, air conditioning, and heat isolation in protective clothes, medical textiles, blankets, bed clothes, bed fabrics, etc. PCMs can be incorporated into polymer solutions or melts during fiber spinning,^6,25–30 as hollow fibers,³¹ or applied to fabrics during finishing processes by conventional the pad–dry–cure method or coating.^3,9,10

In this study, preparation and characterization of microcapsules with surface functional groups were carried out to impart thermal energy storage properties to textiles. They were expected to adsorb onto textile materials easily due to electrostatic interactions. Functional MEPCMs were prepared via copolymerization of methyl methacrylate, acrylic acid, and a crosslinker to form a shell.

Experimental details

Materials

The n-eicosane (Alfa Aesar) with 99% purity was used as received. Methyl methacrylate (MMA, Merck), acrylic acid (AA, Merck), and ethylene glycol dimethacrylate (EGDM, Merck) monomers were washed with NaOH solution before use. Triton X-100 (Merck) was used as surfactant in the synthesis. tert-Butyl hydroperoxide (Merck) was used as received. Ferrous sulfate heptahydrate, ammonium persulfate, and sodium thiosulfate were all obtained from Sigma Aldrich Company and used as received.

Microcapsule preparation

Poly(methyl methacrylate-co-acrylic acid)/n-eicosane (P(MMA-co-AA)/n-eicosane) microcapsules were produced by an oil-in-water emulsion polymerization method. A 100 g quantity of MMA monomer, 100 g of n-eicosane, 20 g of EGDM, and AA monomer (1 g, 5 g, or 10 g for differing AA acid ratios) were assembled as core phase in 400 mL of deionized water. To form an emulsion, 10 g of Triton X-100 surfactant was added and stirred. Then, the mixture was heated to 50℃ under a nitrogen atmosphere and homogenized at 10,000 r/min for half an hour using a mechanical homogenizer. A 1 g quantity of ammonium persulfate ((NH₄)₂S₂O₈) and 8 mL freshly prepared FeSO₄·7H₂O solution were added and the reaction medium was heated to 80℃ to start addition polymerization. Then 0.25 g sodium thiosulfate (Na₂S₂O₇) and 1.00 g 70% tert-butyl hydroperoxide solution were added, and the resultant mixture was stirred for 5 h at 500 r/min. The colloidal emulsion was concentrated by casting water at the end of polymerization. The precipitate was dried under vacuum at 40℃. The recipes and abbreviated names of the microcapsules are listed in Table 1.

Table 1.

MEPCM recipes and abbreviations

MEPCM	Shell recipe of MEPCMs	MEPCM core contents
P(MMA-co-AA)/n-eicosane-1	100 g MMA + 20 g EGDM + 1 g AA
P(MMA-co-AA)/n-eicosane-2	100 g MMA + 20 g EGDM + 5 g AA	100 g n-eicosane
P(MMA-co-AA)/n-eicosane-3	100 g MMA + 20 g EGDM + 10 g AA
P(MMA-co-AA)/n-octadecane-1 ^a	100 g MMA + 20 g EGDM + 1 g AA
P(MMA-co-AA)/n-octadecane-2 ^a	100 g MMA + 20 g EGDM + 5 g AA	100 g n-octadecane
P(MMA-co-AA)/n-octadecane-3 ^a	100 g MMA + 20 g EGDM + 10 g AA
P(MMA-co-AA)/n-hexadecane-1 ^a	100 g MMA + 20 g EGDM + 1 g AA
P(MMA-co-AA)/n-hexadecane-2 ^a	100 g MMA + 20 g EGDM + 5 g AA	100 g n-hexadecane
P(MMA-co-AA)/n-hexadecane-3 ^a	100 g MMA + 20 g EGDM + 10 g AA

P(MMA-co-AA)/n-octadecane and P(MMA-co-AA)/n-hexadecane microcapsules were produced in our earlier studies and are shown here for comparison.^33,34

Characterization of microcapsules

Thermal properties of the MEPCMs such as heat energy storage–releasing capacity and temperature were determined using differential scanning calorimetry (DSC, Perkin-Elmer Jade) at a heating–cooling rate of 10℃/min between −5℃ and 80℃ under a constant stream of nitrogen at a flow rate of 60 mL/min. The encapsulation ratios of the MEPCMs were calculated according to our previous study.³²

Thermogravimetric analysis (TGA) was carried out using a thermal analyzer (Perkin-Elmer TGA7). The TGA instrument was calibrated with calcium oxalate from 25 to 600℃ at a heating rate of 10℃/min in a static air atmosphere. DTG was also obtained to determine maximum rate of weight loss.

The spectroscopic analyses of the microcapsules were performed on KBr disks using an FT-IR spectroscopy instrument. FT-IR spectra of n-eicosane, acrylic acid, methyl methacrylate. and MEPCMs were obtained using a Jasco 430 model FT-IR spectrophotometer between 4000 and 400 cm⁻¹. The number of scans was 16 and resolution was 4 cm⁻¹ during FT-IR analysis.

The morphology of the microcapsules was investigated using a scanning electron microscope (SEM, LEO 440 Computer Controlled Digital). The surface of the microcapsules was coated with gold paint to supply surface conductivity during SEM analysis. The particle size of microcapsules was measured using a particle size instrument (Malvern MS2000E). In the particle size analysis, dried microcapsules were mixed in water and homogenized by a mechanical homogenizer at a rate of 10,000 r/min for 45 min.

Results and discussion

Thermal properties of P(MMA-co-AA)/n-eicosane microcapsules

Energy storage and release capacities of the microcapsules with different copolymer shell structure, i.e. P(MMA-co-AA)/n-eicosane-1–3, are presented in Figure 1. The results obtained from DSC analysis are also summarized in Table 2. As shown in Figure 1(a) and Table 2, the melting and solidifying temperatures of the P(MMA-co-AA)/n-eicosane-1 microcapsule were measured as 31.7 and 33.8℃, respectively, and the latent heats of melting and solidification of the P(MMA-co-AA)/n-eicosane-1 microcapsule were measured as 90.9 and −88.4 J/g, respectively.

Figure 1.

DSC curves of P(MMA-co-AA)/n-eicosane-1 (a), P(MMA-co-AA)/n-eicosane-2 (b), and P(MMA-co-AA)/n-eicosane-3 (c) microcapsules.

Table 2.

DSC results of P(MMA-co-AA)/n-eicosane microcapsules

Microcapsule	Melting enthalpy (J/g)	Melting temp. (℃)	Crystallization enthalpy (J/g)	Crystallization temp. (℃)	Encapsulation ratio (%)^b
P(MMA-co-AA)/n-eicosane-1	90.9	31.7	−88.4	33.8	32.9
P(MMA-co-AA)/n-eicosane-2	77.9	36.1	−67.8	35.6	28.2
P(MMA-co-AA)/n-eicosane-3	50.4	33.6	−49.2	34.3	18.2
P(MMA-co-AA)/n-octadecane-1 ^a	86.1	26.4	−84.4	25.9	34.7
P(MMA-co-AA)/n-octadecane-2 ^a	84.3	26.9	−79.5	26.2	34.0
P(MMA-co-AA)/n-octadecane-3 ^a	82.3	26.9	−77.3	26.1	33.2
P(MMA-co-AA)/n-hexadecane-1 ^a	84.5	16.2	−79.6	15.3	35.6
P(MMA-co-AA)/n-hexadecane-2 ^a	57.2	17.5	−51.4	15.7	24.1
P(MMA-co-AA)/n-hexadecane-3 ^a	71.2	15.1	−50.8	15.4	30.0

The DSC data for P(MMA-co-AA)/n-octadecane and P(MMA-co-AA)/n-hexadecane microcapsules were for comparison to our previous studies.^33,34

Melting enthalpies of n-eicosane, n-octadecane, and n-hexadecane are 276.7 J/g, 247.9 J/g, and 237.3 J/g, respectively.

DSC curves of P(MMA-co-AA)/n-eicosane-2 microcapsules are presented in Figure 1(b). Thermal properties evaluated from the curves indicated that microcapsules melted and solidified at 36.1℃ and 35.6℃, respectively. This is consistent with the melting and solidification temperatures of pristine n-eicosane which were measured as 36.1℃ and 30.6℃, respectively. The latent heats of melting and freezing of P(MMA-co-AA)/n-eicosane-2 microcapsules were measured as 77.9 J/g and −67.8 J/g, respectively.

According to the DSC curves for P(MMA-co-AA)/n-eicosane-3 microcapsules, melting and solidifying transitions occurred at 33.6℃ and 34.3℃, respectively (Figure 1(c)). In addition, the latent heat of melting and freezing of the microcapsules were measured as 50.4 J/g and −49.2 J/g, respectively.

In the MEPCMs, the higher the PCM content, the higher the latent heat storage capacity in the capsules. Encapsulation ratio of n-eicosane decreased as the amount of AA monomer added to copolymers recipe increased. The same trend was also observed in our previous studies,^33,34 as shown in Table 2. It is clear from the DSC analysis that the microcapsules have valuable heat storage capacity. Better results for encapsulation ratios are possible for different acrylic shell capsules.³⁵ Qiua et al. used butyl methacrylate as a co-monomer, which is known to make acrylic structures more elastic. They produced microcapsules with higher enthalpy values.³⁵ In contrast, AA is a hard monomer producing the reverse but functionality at the same time. Therefore, the P(MMA-co-AA)/n-eicosane microcapsules are potential materials to impart thermal comfort properties to textiles. On the other hand, the phase change temperatures of n-eicosane in the microcapsules were slightly different than pristine n-eicosane. Therefore, MEPCMs could be prepared with consistency.

Thermal stability of P(MMA-co-AA)/n-eicosane microcapsules

TGA curves of the P(MMA-co-AA)/n-eicosane microcapsules are shown in Figure 2. The microcapsule system is somewhat complicated due to the fact that two acrylic monomers and a crosslinker are reacted and n-eicosane is included in the core. The crosslinker monomer affects the system over a wide range. It is well known that crosslinked structures do not degrade at definite temperatures; instead they degrade over wide temperature ranges. In the present systems, four-step thermal degradation processes can be expected. The weight loss of the P(MMA-co-AA)/n-eicosane-1 microcapsules was somewhat larger than that of the P(MMA-co-AA)/n-eicosane-2 and -3 samples during first-step thermal degradation processes. This is because the mass of the n-eicosane in the P(MMA-co-AA)/n-eicosane-1 is larger than that in P(MMA-co-AA)/n-eicosane-2 and -3. This is consistent with the DSC data. As shown in Figure 2, the first step occurs at the temperatures between 120 and 230℃, corresponding to the thermal degradation of the paraffin molecular chains.¹⁸ The other steps corresponding to the thermal degradation of the microcapsule shell take place between 220 and 300℃ and are very difficult to be assigned due to undetermined reaction temperatures and overlaps. The P(MMA-co-AA)/n-eicosane-3 microcapsule has the highest ratio of AA in the system and therefore the degradation step of AA is only very slightly observed in this step.

Figure 2.

TGA curves of the P(MMA-co-AA)/n-eicosane-1 (a), P(MMA-co-AA)/n-eicosane-2 (b), and P(MMA-co-AA)/n-eicosane-3 (c) microcapsules.

Chemical characterization of the microcapsules by FT-IR spectroscopy

In this study, methyl methacrylate was polymerized as copolymers of acrylic acid to form P(MMA-co-AA) shell microcapsules with n-eicosane. Carboxylic acid groups of acrylic acid molecules brought about surface functionality to these copolymer capsules. To prove the carboxylic acid content in the capsules, FT-IR spectroscopy was used. For this reason, the spectra of the ingredients and the microcapsules are shown in Figure 3.

Figure 3.

FT-IR spectra of MMA (a), AA (b), n-eicosane (c), P(MMA-co-AA)/n-eicosane-1 microcapsules (d), P(MMA-co-AA)/n-eicosane-2 microcapsules (e), and P(MMA-co-AA)/n-eicosane-3 microcapsules (f).

The following are remarks from the results of FT-IR analysis:

The carbonyl peak of MMA at 1741 cm⁻¹ and AA at 1716 cm⁻¹ overlapped generally in the spectrum of the capsules and emerged at 1731 cm⁻¹. This was proof that the structure contained both a carboxylic acid group and an ester group.

The peaks at 1625 cm⁻¹ and 1635 cm⁻¹ in the spectra of AA and MMA monomers were –C=C– stretching peaks of double bonds and they were invisible in the spectra of microcapsules, which means that polymerization was complete.

The broad peaks observed in the hydrogen bonding region in the spectra of microcapsules were due to –OH stretching band of carboxylic acid groups in copolymer structure.

The characteristic C–H stretching peaks of n-eicosane emerged at 2925 cm⁻¹, 2859 cm⁻¹, 1455 cm⁻¹, and 1380 cm⁻¹ in the FT-IR spectra of the microcapsules. This was proof for the presence of paraffin in the microcapsule structures.

The fabric construction and chemical compatibility of the fabric material with microcapsule shell material can influence the amount of the microcapsules added on the fabric. If the fabrics have sufficient porous construction so that microcapsules can be placed, the heat storage capacity of the fabrics increases. Acid functional groups of P(MMA-co-AA)/n-eicosane microcapsules reveal another reason for microcapsules to be homogeneously and more effectively distributed among the textiles. This is due to hydrogen bonding interactions between the functional group of fibers, such as hydroxyl groups in cellulose, and the acid functional group of microcapsules. The functional moiety of the microcapsules helps their dissolution into application solutions and therefore homogeneous distribution and absorption onto textile material. Functional groups are also very suitable when a chemical binder is used because they interact much better to the binder chemicals.

SEM analysis

Figure 4 shows SEM images and particle size distribution curves of the three P(MMA-co-AA)/n-eicosane microcapsules. It can be observed that all microcapsules had spherical shape and that P(MMA-co-AA)/n-eicosane-1 microcapsules, in particular, had smooth and compact surfaces. According to the SEM images (Figure 4(a)), the capsules were nano-sized and their particle sizes were uniform.

Figure 4.

SEM images (a) and particle size distribution curves (b) of P(MMA-co-AA)/n-eicosane-1, P(MMA-co-AA)/n-eicosane-2, and P(MMA-co-AA)/n-eicosane-3 microcapsules.

Better results are possible concerning microencapsulation research. For example, Zhang et al. achieved a very high ratio of encapsulation with a low rate of emulsification at 400 r/min mixing speed and without using a crosslinker chemical.³⁶ However, the microcapsules were prepared with surface functionality in this work. The AA content affected the emulsion system and changed spherical shapes and particle sizes to larger sizes. This was also observed by optical microscopy.

Particle size distribution analysis

Figure 4(b) shows the particle diameter distribution of the microcapsules. As shown in Figure 4(b), P(MMA-co-AA)/n-eicosane microcapsules had almost unimodal and narrow particle size distribution and the average particle size of microcapsules was between 22.53 µm and 11.73 µm. There is only one major peak in the system with a small tail because of coagulation. Theoretically, a multimodal system cannot be explained if there is no mixture of different products. The samples were prepared in a one-shot process and in the case of mixing the peaks are more likely to be individually separated. As a result, the peaks are considered as representing a unimodal distribution. The average particle sizes of P(MMA-co-AA)/n-eicosane-1, P(MMA-co-AA)/n-eicosane-2, and P(MMA-co-AA)/n-eicosane-3 microcapsules were measured as 22.53 µm, 21.87 µm, and 11.73 µm, respectively. Polydispersity indexes of the particles were also calculated in terms of the relative standard deviation (CV) of the size distributions that were determined using dynamic laser scattering instrument. P(MMA-co-AA)/n-eicosane-1, P(MMA-co-AA)/n-eicosane-2, and P(MMA-co-AA)/n-eicosane-3 microcapsules were almost monodisperse with a polydispersity index of 0.96, 0.73, and 0.92, respectively. According to the results of particle size measurements, the average particle sizes were higher than the particle sizes apparent on SEM images. This is commonly observed for organic compounds because of aggregation of nano-sized capsules.³⁷ The increasing amount of functional groups enhances coagulation due to hydrogen bonding of carboxylic acid groups. On the other hand, carboxylic acid groups help dissociation of microcapsules into application media. Therefore the microcapsules could be possibly applied and homogeneously distributed among textile materials.

Conclusion

In this study, microcapsules with a P(MMA-co-AA) shell and n-eicosane core were produced with the aim of enhancing the thermal comfort of textiles. The microcapsules produced were capable of absorbing 90.4–50.4 J/g and releasing −88.4 to −49.2 J/g of heat. According to the FT-IR results, microcapsules containing variable amount of AA in copolymer structure had functional carboxylic groups on the backbone of the copolymer. The presence of n-eicosane in the microcapsules was also proven by FT-IR spectroscopy analysis and DSC analysis. The data obtained by particle size analysis indicated that the mean particle sizes of microcapsules were between 11.73 and 22.53 µm, exhibiting a narrow distribution. SEM analysis showed that microcapsules had almost a unimodal size distribution and spherical shape. Three types of new group of P(MMA-co-AA)/n-eicosane microcapsules with functional outer surface, considerably high enthalpy, suitable phase change temperature, and particle sizes were produced to be used as thermal comfort additive in textiles. Here it was shown that it is possible to prepare microcapsules with n-eicosane in the core at any ratios of AA. However, the acrylic acid content changes the phase change temperature of n-eicosane in the microcapsules very slightly as it causes big decreases in the encapsulation ratio. Therefore, it can be concluded that the ratio of acrylic acid should be as low as possible to obtain high ratios of encapsulation. On the other hand, it should be sufficiently available in the system for surface functionality. The results are consistent with the conclusions of our previous studies.

Footnotes

Funding

This work was supported by the Scientific and Technological Research Council of Turkey (TUBITAK) (grant number 111M484).

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