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Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. REFERENCES

PRS® paraffin wax was encapsulated by means of suspension-like copolymerization of methyl methacrylate (MMA) with butyl acrylate (BA). The effects of the polymeric shell dry glass transition temperature (Tg) and the reaction temperature (Tr) were then studied. Additionally, the evolution of particle diameter, molecular weight, conversion, and Tg during polymerization was also researched. The chemical properties of the shell material (acrylic polymer), together with those found in the core material (PRS® paraffin wax), for instance: polarity and interfacial tensions, largely determine whether the morphology of the microcapsules will be thermodynamically favored or not. The high polarity of MMA (γ0 = 18 mN m−1) and BA (γ0 = 24 mN m−1) should provide a thermodynamic driving force to cover the paraffin wax droplet which would result in a core/shell thermodynamically favored structure. However, most systems are defined by kinetics rather than thermodynamics such as the monomers dry Tg and Tr. It was observed that penetration of polymer radical chains was severely limited when the dry Tg was ≥10°C above the reaction temperature, resulting in irregular and undifferentiated particles. However, penetration did occur when the copolymeric shell dry Tg was ∼10°C below the reaction temperature which led to uniform and spherical particles being synthesized. POLYM. ENG. SCI., 54:208–214, 2014. © 2013 Society of Plastics Engineers


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. REFERENCES

Nowadays, the rise in world energy demand calls for new energy solutions. Furthermore, conventional fossil energy sources are limited and environmentally unsustainable due to harmful greenhouse gases. In this context, thermal energy storage has proved to be important technology for use in renewable energy systems and has received growing attention in recent years [1, 2].

Phase change materials (PCM) are a series of functional materials capable of storing and releasing energy when undergoing a phase transition. PCMs absorb heat when they melt (phase change from solid to liquid) and release it when they solidify (phase change from liquid to solid). Some applications they have, described elsewhere, are: thermal storage of solar energy [3], cooling [4], thermal protection of food [5], for use in the textile industry [6] and in energy conservation in buildings with thermal comfort [7, 8]. Some of the more commonly used PCMs are linear chain hydrocarbons known as paraffin waxes (or n-alkanes), hydrated salts, polyethylene glycols (PEGs), fatty acids and mixtures or eutectics of organic and nonorganic compounds. Commercial paraffin waxes compared to others PCMs are widely available, have a moderate thermal storage density of (200 kJ kg−1), a wide range of melting temperatures and are chemically inert and inexpensive [9].

Moreover, they can be encapsulated with a plastic or crosslinked polymer shell to prevent liquids from leaking during their use. The most common methods for paraffin microencapsulation are interfacial polymerization [10, 11], emulsion polymerization [12, 13], in situ polymerization [14, 15], layer by layer deposition of polyelectrolyte [16, 17], coacervation [18], spray drying [19] and suspension polymerization [20, 21].

The choice of shell material is one of the most important aspects of the process for controlling the mechanical strength, thermal properties and the microcapsules morphology and the most widely used materials to encapsulate paraffin waxes are gelatine and arabic gum [22, 23], melamine-formaldehyde prepolymer [24, 25], silica [26] and methyl methacrylate-based polymer [27, 28].

In previous research [21, 29], PCMs were successfully encapsulated in a polystyrene shell using a suspension-like polymerization technique. However, cross-section micrographs of the poly(methyl methacrylate-co-styrene) revealed a composite salami-like internal morphology (as opposed to a more traditional core/shell) due to the lack of a strong driving force for phase separation of the polystyrene formed within the paraffin wax droplet. The complex interaction between the polymer system (initiators, monomers, and polymers) and the continuous phase (usually water and the suspension agent) on the surface of the droplets is responsible for the success in the encapsulation. Thus, the chemical property differences between the encapsulating “shell” material, and the “core” material, such as polarity and interfacial tensions, determined to a large extent whether the morphology of the microcapsules would be thermodynamically favored or not. Here, the polystyrene polarity value was quite similar to that of the paraffin wax, and thus the core/shell morphology was not thermodynamically favored during polymerization [21].

The paraffin wax was also microencapsulated by means of a suspension-like polymerization using a shell based on a methyl methacrylate and styrene copolymer [30]. The average energy storage capacity (87.5 J g−1) of the microcapsules obtained using this copolymer was higher than the values reported in previous papers for styrene alone (41.7 J g−1). Hence, the higher polymerization rate and polarity of methyl methacrylate compared to styrene favored the development of a core/shell morphology. Moreover, a higher latent heat value was achieved.

Additionally, the microcapsules containing PCMs were synthesized using acrylic-based polymer shells to improve the paraffin encapsulation efficiency. The acrylic shell composition was modified so as to achieve greater encapsulation efficiency (93.5%) and a higher latent heat value (94.8 J g−1), which was more than double that of the value obtained with a polystyrene shell [31].

Copolymerization of methyl methacrylate with butyl acrylate (P(MMA-co-BA)) as shell material should form a thermodynamically favored core/shell morphology. The greater combined polarity of MMA (γ0 = 18 mN m−1) and BA (γ0 = 24 mN m−1) in comparison with styrene (γ0 = 32 mN m−1) [32] should provide a thermodynamic driving force to cover the paraffin wax droplet.

However, Sundberg et al. [33] stated that the majority of systems are defined by kinetic factors such as the monomer dry glass transition temperature (Tg) and the reaction temperature (Tr) and with these considerations, PMMA-based paraffin encapsulation should have produced microcapsules with completely separated phases. However ESEM micrographs showed there to be a uniform mixture of both paraffin wax and PMMA polymer domains [31]. This was probably due to the fact that the wet polymer glass transition temperature (94°C) was markedly higher than the reaction temperature (70°C) and, as a result, mobility of the polymeric radicals was low due to their glassy state. Thus, copolymerization of MMA with BA, whose Tg is −40°C [32], would decrease the copolymeric shell Tg. According to Sundberg et al. [34], the penetration of polymer radical chains fully occurs when the copolymeric shell dry Tg is about 10–15°C below the reaction temperature.

In this article the effect of Tg on the microencapsulation process was evaluated by varying the copolymeric shell dry Tg. Alay et al. [35] synthesized microcapsules by using emulsion polymerization of poly(butyl acrylate) (PBA) for a shell and n-hexadecane for a core. Zhang et al. [36] microencapsulated paraffin wax with poly (methyl methacrylate-co-acrylic acid) for a shell by means of emulsion polymerization.

The evolution of particle diameter, molecular weight, conversion, and dry glass transition temperature during the microencapsulation process was then studied so as to achieve a better understanding of the microencapsulation process.

EXPERIMENTAL

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. REFERENCES

Materials

The monomers, methyl methacrylate (99 wt%, Merck Chemical) and butyl acrylate (99 wt%, Panreac Chemical), were used. Butyl acrylate and methyl methacrylate were washed with sodium hydroxide to remove the inhibitor. Calcium chloride was then used as a desiccant, benzoyl peroxide (97 wt%) was the initiator (Fluka Chemical) and commercial grade PRS® paraffin wax (Repsol YPF) was the oily core. This paraffin was a mixture of hydrocarbons C19-C27 with an energy storage capacity of 202.7 J g−1 and a melting temperature ranging from 40 to 45°C, produced and sold by the petrochemical company Repsol YPF (Spain). Polyvinylpyrrolidone (K30, Mw 40,000 g mol−1) (Fluka Chemical) was the stabilizer and methanol was used to pour the samples. The reagents were used as received. Water was purified by distillation and subsequent deionization using ion-exchange resins. Finally, high-purity grade nitrogen was used.

Microcapsule Synthesis

Suspension-like polymerization reactions were carried out in a 0.5-L double-jacketed glass reactor and in a 10-L double-jacketed glass reactor equipped with the stirring rate and temperature digitally controlled, a reflux condenser and a nitrogen gas inlet tube. This experimental setup and the suspension-like polymerization technique were described previously in a schematic diagram [21]. There were two phases in the synthesis process: (i) a continuous phase containing water and polyvinylpyrrolidone and (ii) a discontinuous phase containing butyl acrylate, methyl methacrylate, PRS® paraffin wax and benzoyl peroxide.

First, the continuous phase was transferred to the glass reactor mildly agitated (150 rpm). Then, the initiator was premixed with monomers and the phase change material. Next, the discontinuous phase was added to the continuous phase and was kept at agitation of 500 rpm at a constant temperature of 70°C. Polymerization was then carried out for 5 h with a nitrogen atmosphere. Once obtained, the PCM microcapsules were repeatedly washed with methanol and filtered to remove impurities and unencapsulated paraffin wax. The purified microcapsules were then dried at room temperature. Finally, the conversion was calculated gravimetrically.

Environmental Scanning Electron Microscopy (ESEM) Observation

The surface and internal morphological features of the microcapsules after polymerization were observed by using a XL30 (LFD) ESEM. A cross section of the microcapsules to reveal the internal microstructure was obtained at room temperature by using an Ultramicrotome Leica EM UC6 diamond blade, while samples were observed with an optical microscope.

Number-Average and Volume-Average Diameter of the Microcapsules

Particle size and particle size distribution (PSD) were determined with a Malvern Mastersizer Hydro 2000 SM light scattering apparatus, prior dilution of the particles in methanol.

Differential Scanning Calorimetry (DSC) and Modulated DSC (MDSC)

The melting point and heat of fusion of the various materials used and obtained were determined using a modulated differential scanning calorimeter (DSC), TA Instruments model Q100. The DSC was equipped with a refrigerated cooling system and nitrogen was used as the purge gas. DSC measurements were carried out by heating from −30 to 80°C for two cycles with a 10°C min−1 heating rate to observe the melting transitions and corresponding enthalpy. Each sample was analyzed at least twice, and the average values obtained were recorded. This procedure was carried out for both the pure paraffin wax and the paraffin/acrylic composite capsules.

For the measurement of dry glass transition temperature (Tg), samples were subjected to an equilibrate at 80°C for 1 min followed by two successive heating steps ranging from −20 to 120°C. This DSC measurement was run in modulated mode where the heating ramps used a modulation amplitude of ±3°C, with 60-s period and an underlying heating rate of 3°C min−1. The microcapsules synthesized were analyzed twice to ensure reliable results.

Gel Permeation Chromatography (GPC) Measurement

The molecular weight distribution (MWD) of the polymer which made up the encapsulating shell was measured by gel permeation chromatography (GPC) using a chromatograph model 150-GPCV LC from Waters (USA). Tetrahydrofuran (THF) was used as the elution solvent. Poly(styrene) standards from Waters were used for MWD calibration.

RESULTS AND DISCUSSION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. REFERENCES

Influence of the Dry Glass Transition Temperature (Tg) and Reaction Temperature (Tr) on the Microencapsulation Process

PRS® paraffin wax was microencapsulated by means of a suspension-like polymerization using a shell based on a methyl methacrylate (MMA) and butyl acrylate (BA) copolymer.

Table 1 shows the interfacial tension (γ0) and the dry glass transition temperature (Tg) of methyl methacrylate and butyl acrylate. Then, to study how the polymeric shell dry glass transition temperature (Tg) affected the microencapsulation process, the copolymer Tg was varied taking into account the Fox equation:

  • display math(1)

where w1 and Tg1 correspond to weight fraction and PMMA glass transition temperature respectively, and w2 and Tg2 are weight fraction and PBA glass transition, respectively.

Table 1. Properties of methyl methacrylate and butyl acrylate [32]
Polymerγ0 (mN m−1)Tg (°C)
MMA18119
BA24−40

To carry this out, four experiments were made by varying the shell composition whilst the reaction temperature (Tr) was kept constant at 70°C (Table 2).

Table 2. Experiments carried out using different dry glass transition temperatures
Exp.Monomers proportionsPolyvinylpyrrolidone/ monomers (g/g)PRS® paraffin wax/monomers (g/g)Stirring rate (rpm)Tr (°C)Tg (°C)
168.7%MMA/31.3%BA0.09431.025007050
274.0%MMA/26.0%BA0.09431.025007060
383.8%MMA/16.2%BA0.09431.025007080
488.3%MMA/11.7%BA0.09431.025007090

Shell polarity was increased from experiment 1 to 4 because MMA (γ0 = 18 mN m−1) is more polar than BA (γ0 = 24 mN m−1) and according to Sundberg et al. [37] a polar shell should form microcapsules with a core/shell structure due to phase separation is favored. In a similar fashion, the dry glass transition temperature also increased as MMA has a higher dry Tg than BA (Table 1).

Table 3 summarizes the initial visual presentation and the experimental Tg values obtained by MDSC analysis of the microcapsules obtained using different proportions of monomers. The results from the MDSC analysis indicated that the experimental and theoretical values were, to a large degree, in accordance. Therefore, all the copolymer chains prepared by suspension polymerization were random and homogeneous.

Table 3. Schematic particle morphologies achieved using different monomers proportions
Exp.Monomers proportionsTr (°C)Tgtheoretical (°C)Tgexperimental (°C)Morphology descriptionSchematic particle morphology
  1. Black color is acrylates polymer. Gray color is paraffin wax.

168.7%MMA/31.3%BA705051.7Uniform and spherical particlesimage
274.0%MMA/26.0%BA706061.5Uniform and spherical particlesimage
383.8%MMA/16.2%BA708081.2Irregular particlesimage
488.3%MMA/11.7%BA709091.4Large irregular particlesimage

As shown in the table, the thermodynamically most favored structure (Exp. 4) was that which provided the most irregular microcapsules. It seems evident that microcapsules morphology is kinetically controlled rather than thermodynamically which is in accordance with Sundberg and Stubbs [33] who stated that most systems are produced under the former. These authors considered three decisive kinetic factors and put forward that if one of these is restrained by diffusion kinetics, then, morphology equilibrium will not be attained so, kinetically controlled systems with unbalanced structures will be obtained.

These three main kinetic factors that Sundberg et al. [33] observed are the following:

  1. Penetration of polymer radical chains into the particle interior after entry from the water phase.
  2. Phase separation of immiscible polymer chains produced at the different polymerization stages.
  3. Spatial rearrangement of phase separated domains.

The methyl methacrylate reaction rate and its high dry Tg value, in comparison with butyl acrylate, caused that the copolymer dry glass transition can be higher than the reaction temperature. This produced a fast solidification rate, and so, differentiated particles were not yielded (experiments 3 and 4). However, the shell compositions used in these experiments should have been favorable, at least from a thermodynamic viewpoint, for yielding core/shell structures. Therefore, it seems apparent that kinetic factors have a greater overall effect on encapsulation than thermodynamic ones.

On the other hand, in experiments 1 and 2 an acrylic shell composition was used that provided a dry Tg value which was lower than the reaction temperature (70°C). However, the particles synthesized in experiment 2 showed to have a more uniform and spherical morphology than those obtained in experiment 1 (Tg = 50°C). This fact could be attributed to the shell polarity for experiment 2 is more polar than the one obtained in experiment 1, then according to Sundberd et al. [37] the resulting morphology should be a core/shell. Sundberg et al. [38] established that polymer radical chains penetration occurred fully (that is, they could reach the particle center) when the copolymeric shell dry Tg was about 10–15°C below the reaction temperature (Exp. 2: Tr = 70°C; Tg = 60°C). Nevertheless, penetration was severely limited when this was at least 10°C above mentioned reaction temperature (Exp. 3 and 4). Logically, the extent of radical penetration was expected to be partial at any point within this range. The conclusion drawn from this is that the optimum difference between the reaction temperature and the dry Tg must be higher than 10°C.

Elemental analyses by energy dispersive X-ray spectroscopy (EDS) were then carried out on the cross-sectioned ESEM images to determine the average composition of the core and shell regions of the microparticles obtained in experiment 2. The EDS showed the cores were on average 89.5 ± 0.5 wt% of carbon and 10.5 ± 0.5 wt% of oxygen whereas the shells were on average 76.8 ± 0.5 wt% of carbon and 23.2 ± 0.5 wt% of oxygen. These results were in keeping with the fact that the inner structure of the particles were made up of a mixture of both the paraffin matrix and P(MMA-co-BA) domains, whereas the outer shell was predominantly made up of P(MMA-co-BA).

How the Reaction Evolved Throughout Suspension-like Polymerization

The evolution of the polymerization rate, particle diameter, molecular weight and dry Tg during suspension polymerization was studied using a pilot plant (10-L glass reactor). Taking into account the results obtained in the previous study, the polymeric shell dry Tg was fixed at 60°C (Exp. 2) as at this temperature uniform and spherical microcapsules were produced.

Particle Diameter Evolution

Throughout polymerization different samples were taken with the purpose of analyzing how the particle diameter had evolved and the evolution of average diameters in volume (dpv0.5) and in number (dpn0.5) together with the reaction time is illustrated in Fig. 1.

image

Figure 1. Average volumetric and numeric diameters evolution of microcapsules obtained by suspension like polymerization. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary. com.]

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According to Jahanzad et al. [39, 40] the balance between the drop breakup rate (Rb) and coalescence (Rc) determines the particle droplets size. These rates are affected by several parameters, namely: density, viscosity, interfacial tension, suspending agent, and stirring rate. As expected, the mechanism of microcapsules formation was quite similar to that found in suspension polymerization and Table 4 summarizes the particle formation stages during microencapsulation.

Table 4. Particle formation stages on the microencapsulation process
StageRb vs. RcResultTime (min)
  1. Rb is drop breakup rate; Rc is coalescence rate.

TransitionRb >> RcDecrease in drop size0–30
Quasi-steady-stateRbRcAlmost constant drop size30–90
GrowthRb << RcIncrease in drop size90–120
IdentificationRb = Rc = 0Final particle size achieved120–180

According to Jahanzad et al. [40], during suspension polymerization four stages can be identified. In their experiments, reaction times obtained by these authors were lower than the ones attained in this research, which can probably be put down to the polymeric shell composition. Jahanzad et al. [40] carried out their experiments using methyl methacrylate instead of using copolymerization of methyl methacrylate with butyl acrylate, but it was observed that the butyl acrylate polymerization rate was lower than that of methyl methacrylate and, as a result, the reaction times obtained in this article were higher.

Molecular Weight Evolution

The evolution of molecular weight (Mw) and polydispersity (PDI) together with the conversion rate is depicted in Fig. 2.

image

Figure 2. Evolution of Mw and PDI of microcapsules obtained by suspension like polymerization. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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As shown in Fig. 2, at the beginning of the suspension polymerization reaction, the molecular weights obtained were very high (t = 10 min → Mw = 68,844 g mol−1). This was likely due to the most volatile components evaporating before the gel permeation chromatography analysis was carried out. In addition, a progressive upward trend in molecular weight was observed for up to 120 min, but from then onwards a high molecular weight was maintained constant (≈104,000 g mol−1). This state is referred to in the literature on the topic as the identity point (IP), as it is the point where the microcapsules at last reach their final diameter and molecular weight.

Conversion Evolution

Figure 3 shows how conversion (%) and average volumetric diameter (dpv0.5) evolved. It can be observed that the former remained practically constant with a value of ∼12% up to 45 min of reaction time. However, between 45 and 90 min, it rose with an ever-increasing tendency attaining a value of 53.7%. Total conversion was obtained at 120 min. At that time the particles acquired their IPs and began acting as solids and, consequently, high viscosity in the reaction medium was attained.

image

Figure 3. Conversion and average volumetric diameters evolution of microcapsules obtained by suspension like polymerization. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary. com.]

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In suspension polymerization, the identity point is defined as the instant in time in which particle diameter begins to be constant. If we consider this parameter, two kinds of behavior can be identified. First, there are systems in which definitive particle diameter is attained at a low conversion (A type) and, second, there are other systems in which they attain their identity point at high conversions (B type). As can be appreciated in Fig. 3, the system behavior analyzed corresponded to the latter. These results were previously reported by Cordoví et al. [41, 42] and Sánchez et al. [30].

Dry Glass Transition Temperature Progress

The dry glass transition temperature was attained only after 120 min as until that moment the particles had not reached their identity points. The logical conclusion from this seems to be that the identity point directly corresponds to the dry glass transition temperature.

From the results obtained, the conclusion can be drawn that the dry glass transition temperature remained unchanged throughout polymerization. However, said temperature was only reached once the particles had acquired their identity points and, consequently, attained full conversion. Moreover, at this stage, the particles reached their final diameter and molecular weight, as can be seen in Figs. 1 and 2 and at the same time viscosity was extremely high so the microcapsules began acting as solids.

CONCLUSIONS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. REFERENCES

Microparticles containing PRS® paraffin wax with a methyl methacrylate (MMA)—butyl acrylate (BA) copolymeric shell were successfully synthesized by means of suspension-like polymerization. The effects of the dry glass transition temperature and reaction temperature were then studied. Irregular particles were observed when the dry Tg was at least 10°C above the reaction temperature, but, uniform and spherical particles were attained when the copolymeric shell dry Tg was about 10–20°C below the reaction temperature. Next, the evolution of particle diameter, molecular weight, conversion rate and dry glass transition temperature during microencapsulation was studied and it was observed that definitive particle diameter was reached at high conversions (B type system). Another noteworthy observation is that dry glass transition temperature did not evolve during polymerization and was only reached with extremely high viscosity and after the particles had attained their identity points (final diameter and molecular weight) and, consequently, had been fully converted.

REFERENCES

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. REFERENCES