Preparation of Microcapsules Containing Benzoyl Peroxide Initiator

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Preparation of Microcapsules Containing Benzoyl Peroxide Initiator with Gelatin-Gum Arabic /PolyureaFormaldehyde Shell and Evaluating Their Storage Stability Maryam Raeesi, S. Mojtaba Mirabedini, and Ramin R. Farnood ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 26 May 2017 Downloaded from http://pubs.acs.org on May 26, 2017

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ACS Applied Materials & Interfaces

Preparation of Microcapsules Containing Benzoyl Peroxide Initiator with GelatinGum Arabic /Polyurea-Formaldehyde Shell and Evaluating Their Storage Stability

M. Raeesia, S.M. Mirabedinia,b*, R.R. Farnoodb a b

Iran Polymer and Petrochemical Institute, P.O. Box 14965-115, Tehran, Iran

Department of Chemical Engineering and Applied Chemistry, University of Toronto, Canada

ABSTRACT This work involves the optimized preparation and characterization of microcapsules which contain benzoyl peroxide (BPO) dispersed in dibutyl phthalate (DBP) with gelatingum arabic (Gel-GA)/Polyurea-formaldehyde (PUF) shell. The microcapsules were prepared in two steps using complex coacervation and in situ polymerization techniques, respectively, at various mixing speeds and different core:shell ratios. The scanning electron microscopy (SEM), optical microscopy, and Fourier transform infrared (FTIR) spectroscopy were used for characterization of prepared microcapsules. The resultant microcapsules were spherical with average diameters about 120-200 µm, had no intercapsule bonding and with thickness of 0.7 to 1.5 µm. The results revealed high core content loading, 82-89 wt % for microcapsules prepared at various mixing speeds. The differential scanning calorimetry analysis (DSC) indicated that the encapsulated BPO was not influenced by encapsulation process and maintained its activity. Moreover, with a compact and double Gel-GA/PUF shell, the microcapsules were stable and no leakage of core material in an acrylate-based resin and toluene as an organic solvent was recorded. The resultant microcapsules have the potential of usage in industries such as self-healing systems and structural adhesives where the impermeability of microcapsules is an important factor.

KEYWORDS: microencapsulation, benzoyl peroxide, gelatin, polyurea-formaldehyde, stability *: Corresponding Author: Tel: +98 21 4866 2401, fax: +98 21 4458 0023, E-mail addresses: [email protected] & [email protected] (S.M. Mirabedini)

1 ACS Paragon Plus Environment


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INTRODUCTION Microencapsulation as a method for protection of active materials from surroundings is commonly used in various applications such as self-healing systems,1,2 pesticides,3 drug deliveries,4 and adhesives.5,6 In particular, in monomer/initiator adhesive systems where early polymerization due to pre-activation of initiator limits the storage time of product,7 microencapsulation could significantly increase the product stability and shelf life. By encapsulating active materials such as initiators and separating them from monomers, polymerization can only occur upon the breakage of microcapsules and the release of encapsulated materials.8 The above strategy could be used to improve the stability of monomer/initiator adhesive systems containing organic peroxides. However, peroxides are known to have severe thermal sensitivity; therefore, the microencapsulation must be performed in a way that it does not adversely affect the chemical activity of these compounds. McFarland et al.7 prepared microcapsules containing cumene hydroperoxide as the core and polyurea as the shell. They reported extended pot life from hours to weeks in certain acrylate systems. Caruso et al.9 reported preparation of urea formaldehyde-based microcapsules containing benzoyl peroxide (BPO) dissolved in phenyl acetate as the core materials. Microencapsulation of melamine formaldehyde containing BPO in toluene (10 wt %) throughout in situ polymerization was also reported by Zhang et al.10 A necessary feature of microcapsule shell used in the adhesive is its impermeability to core (encapsulated) materials, since even a slight leakage of the core material could lead to the premature monomer polymerization. This may be addressed by using multiwalled microcapsules with increased shell compactness and thickness. Li et al.11 reported the preparation of polyurea (PU)/urea-formaldehyde (UF) double-walled


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microcapsules in a two-step procedure through interfacial polymerization and in situ polymerization. More recently, PU/UF double-walled microcapsules have been synthesized by a single-step procedure using ethyl phenyl acetate and isophorone diisocyanate (IPDI) cores.12,13 Yang et al.14 reported the preparation of organic/inorganic hybrid microcapsules with improved thermal stability using polyurea (PU)/poly melamine formaldehyde (PMF) double-layer shells via O/W Pickering emulsion templates. Kang et al.15 introduced triple-walled microcapsules by in situ polymerization of dopamine monomer on exterior layer of as-prepared PU/UF microcapsules. The polydopamine (PDA) layer significantly increased thermal stability, chemical (acid-base) stability, and organic solvent stability of microcapsules. In this paper, a versatile method for preparing double-walled microcapsules with high stability is presented. Among all peroxides, BPO as an appropriate free-radical initiator,16 was encapsulated in a two-step facile process by means of complex coacervation method followed by in situ polymerization. In the first step, the interior shell was formed via complex coacervation of gelatin (Gel) in the presence of gum arabic (GA). Since Gel is a hydrophilic polymer with good film forming properties, Gel-GA layer is expected to have low permeability toward small organic molecules.17 In the second step, via in situ polymerization between urea-formaldehyde (UF) prepolymer and Gel-GA layer, an exterior layer is deposited on the shell in acidic media. The resultant Gel-GA/PUF double-walled microcapsules are subsequently examined for application in acrylate-based adhesives.

EPERIMENTAL SECTION Materials



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Gelatin (Gel, type A, acid processed) used as the shell material was obtained from Sigma-Aldrich. Dibutyl phthalate (DBP), benzoyl peroxide (BPO), gum arabic (GA), hydrochloride acid, sodium hydroxide, urea, 37 wt % formaldehyde solution, Sodium dodecyl sulfate (SDS), polyvinyl alcohol (PVA), ammonium chloride, isopropanol, potassium iodide, glacial acetic acid, sodium thiosulfate, toluene, and ethanol were purchased from Merck Chemical Co. and used as received without any further purification. Degaroute 465 as a methyl metacrylate-based resin (PMM-based) was obtained from Evonik Industries AG.

Synthesis of microcapsule Gelatin-gum

arabic

(Gel-GA)/polyurea-formaldehyde

(PUF)

double-walled

microcapsules were prepared via complex coacervation18 method followed by in situ polymerization in a solid-in-oil-in-water (S/O/W) dispersion. In the first step, different amounts of BPO were well dispersed in DBP (40 wt % BPO paste) under mechanical agitation (Heidolph RZR 2102). Various core:shell ratios (55:45, 60:40, and 65:35) were gently dispersed in 100 mL of Gel aqueous solution (1 wt %) at 45 C ̊ under mechanical agitation at various mixing speeds of 500, 700, and 900 rpm to form a stable dispersion. 100 mL of GA aqueous solution (1 wt %) was then added into the above-mentioned dispersion. To induce complex coacervation, pH was set at 4 using a few droplets of 0.1 N hydrochloride acid solution. Afterwards, the reaction media was cooled slowly from room temperature to 5 ̊C over a period of 40 min with continuous vigorous stirring. After 90 min, pH was increased to 7 by adding several droplets of dilute sodium hydroxide, and then 50 wt % glutaraldehyde solution (2 mL) was added to the mixture. Mechanical stirring was reduced to 400 rpm and the mixture was continuously stirred overnight.


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In the second step of synthesis procedure, the exterior layer of the microcapsule shell was formed following a modified version of the procedure reported by Behzadnasab et al.19 In this step, 5 wt % PVA solution (3 mL), 1 wt % SDS solution (1 mL), urea (1 g), and ammonium chloride (0.15 g) were added to the mixture prepared in the previous step. Then pH was adjusted to 3.5 by adding a few droplets of 0.1 N hydrochloride acid solution. Then 3 g of a 37 wt % formaldehyde solution was gently added to the mixture, and temperature was gradually increased to 55 C ̊ . The process was continued for 4 h, and the resultant microcapsules were separated from the mixture, washed with distilled water three times, and dried at ambient temperature. Optimal conditions (core:shell ratio and mixing speed) for preparation of the microcapsules were considered to be those in which the microcapsules had free flowing characteristics.

Encapsulation Efficiency and Core Content Determination Encapsulation efficiency was calculated by dividing the weight of the resultant microcapsules by the initial weight of all reactants (Gel, GA, DBP, BPO, urea, and formaldehyde). Core content (BPO dispersed in DBP) of the microcapsules was determined using extraction method.20 To this end, the desired amount of microcapsules (W1) was poured into 10 mL of toluene as solvent, and exposed to ultrasound (Bandelin, SONO PLUS-UW 2200) for 10 min with 50 W input power, 20 kHz frequency, and 0.7 s and 0.3 s on and off pulse, respectively. The resultant suspension was passed through the filter paper to separate shell particles, several times washed with toluene, and then dried in oven at 50 ̊C. Each measurement was carried out in triplicate. The Filtrated was weighed (W2) after cooling in a desiccators, and finally core content of the microcapsules was calculated according to: Microcapsules core content (%) = [(W1 – W2)/W1] × 100



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Iodometeric titration method was used for determination of BPO inside the microcapsules, based on the procedure described by Katnur and co-worker.21 For this purpose, 25 mL of isopropanol alcohol was added to 250 mg of crushed microcapsules, and then 10 mL of saturated potassium iodide and 1 mL of glacial acetic acid were added to the mixture. The mixture was stirred for 2-5 min at 60 ̊C and was titrated with 0.1 N sodium thiosulfate without cooling. The amount of BPO was subsequently calculated using:

Active oxygen content (%) = (8VNF/1000W) × 100 Assay peroxide (w/w %) = Active oxygen content × (M/16)

where, V, N, F, W and M stand for volume (mL) of sodium thiosulfate used in titration, normality of sodium thiosulfate, sample’s weight (g), and molecular mass of BPO (g/mol), respectively. All measurements were performed in triplicate.

Characterization The shape and morphology of the resulting microcapsules, after first and second steps of synthesis, were characterized using an Olympus SX optical microscope equipped with a Canon Powershot SX40 digital camera. The surface morphology of the microcapsules was evaluated using a scanning electronic microscope (SEM) (Tescan, Vega II, Czech) operated in the secondary electron mode at 20 kV. The mean diameter of the microcapsules was analyzed through at least 100 measurements using Image J software. In addition, SEM was used for the determination of morphology and thickness of the microcapsules’ shell synthesized at different mixing speeds. For this purpose, the microcapsules were ruptured in liquid nitrogen with a razor blade and sputter coated


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with a thin layer of gold (∼ 10 nm) under argon atmosphere prior to the measurements. In order to better capture statistical variations in shape and morphology of the microcapsules, a total of 10 SEM images were acquired for each sample. Laser particle size analyzer (CILAS 1064 liquid) was utilized for the determination of average particle size and size distribution of the microcapsules at different mixing speeds. The microcapsules (1 g) were dispersed in 200 mL of distilled water by stirring for 10 min and then were placed in the sample holder for particle size measurement. All measurements were performed in triplicates, and the average values were reported. Fourier transform infrared (FTIR) spectroscopy was carried out using PerkinElmer Spectrum One spectrometer in diffuse reflectance mode for the microcapsules and in transition mode for the shells and core contents using potassium bromide (KBr) pellets. Spectra were collected with 16 scans at a resolution of 4 cm-1 and a spectral range of 400-4000 cm-1.

Activity of Encapsulated BPO The reactivity of microencapsulated BPO was studied using differential scanning calorimetry (DSC) technique (Netsch, 200-F3 Maia, Germany). For this purpose, a defined amount of either crushed microcapsules (3.5 wt %) or neat BPO powder (1.5 wt %) was dispersed into PMM-based resin, degaroute 465 (98 wt %). Then the specimens were crimple-sealed in aluminum crucibles, placed immediately in the aluminum DSC pan, and thermographs were recorded in isothermal mode at room temperature. In addition, the viscosity of PMM-based resin containing crushed microcapsules was monitored using a cone & plate rheometer (Paar-physica, MCR 501, Germany) for about 1 h under 200 rpm at 25 ̊C.



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Stability of Microcapsules Stability of prepared microcapsules in PMM-based resin during 2 months was studied using rheometery test and optical microscopy. To this end, the desired amount of the microcapsules synthesized at different mixing speeds was dispersed in 100 g of PMM-based resin, and the viscosity was measured using a cone and plate rheometer during a two months period. The test was performed at 25 ̊C and under 200 rpm. For visual inspection, an aliquot was collected after two months and morphological changes of the microcapsules were investigated by optical microscopy. In order to study the stability of microcapsules in toluene as an organic solvent, 0.5 g of the microcapsules prepared at different mixing speeds was added into toluene, and after 3 days the shape and morphology of the microcapsules were observed using optical microscopy. The release behavior of BPO and DBP from the microcapsules immersed in toluene was quantitatively studied using UV spectroscopy and weight loss method. To this end, a defined amount of microcapsules was immersed in toluene, and after 3 days the microcapsules was separated from the suspension. The filtered microcapsule was dried at 50 ̊C and weighed accurately to determine the weight loss. The toluene content of eluent was evaporated completely under N2 atmosphere. Then the sediment was diluted with ethanol and analyzed by UV spectrophotometer (BEL, LGC 53, Italy). To draw the standard curve, different concentrations of BPO and DBP solutions in ethanol (2-15 ppm) were prepared separately. The absorbance of standard solution of BPO and DBP were measured by UV spectrophotometer at λmax of 230 and 273 nm, respectively.

RESULTS AND DISCUSSION


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The synthetic procedure used in this study for preparing Gel-GA/PUF microcapsules containing dispersed BPO in DBP is schematically presented in Figure 1. In this procedure, to achieve high BPO loadings (up to 40%) inside the microcapsules, BPO was dispersed in the DBP plasticizer rather than being dissolved in an organic solvent. In addition, gelatin solution as an effective protective colloid was able to prohibit the aggregation of BPO particles and hence, produced a stable dispersion. During the first step of the microcapsule synthesis, the addition of BPO paste to the aqueous solution containing Gel and GA and the subsequent pH reduction resulted in the interaction between positively charged Gel and the negatively charged GA molecules. Reducing the temperature induced complex coacervation, and the coacervate phase coagulated at the interface between the organic phase and aqueous phase.

Figure 1. Schematic representation of the synthesis procedure of Gel-GA/PUF double-walled microcapsules containing BPO paste via complex coacervation and in situ polymerization route.



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Adding glutaraldehyde also resulted in the partial cross-linking of Gel and therefore, increased the stability of Gel-GA layer. In the second step of microcapsule synthesis, UF pre-polymer containing amine and hydroxyl groups reacted with the amine and hydroxyl group of Gel-GA layer in acidic media through a condensation reaction (Figure 2). In this way, the second layer PUF can be deposited on the Gel-GA first layer.

Figure 2. Possible reaction between Gel and UF pre-polymer in acidic media

Optimized core:shell ratio for the microcapsules was considered as a ratio in which resultant microcapsules have completely free flowing characteristic with a non-tacky surface. Stickiness is a result of the release of the core materials to the outside of microcapsules. Overall microencapsulation yield, core content, and BPO content of the microcapsules in optimized condition were depicted in Table 1. As it can be seen, microcapsule yield of 55 to 65 wt % was achieved for different mixing speeds. By increasing mixing speed, the overall yield decreased. The core content also decreased from about 88.7 wt % to about 81.8 wt % as the mixing speed increased from 500 to 900 rpm. This is expected that with increasing mixing speed, the average microcapsule size and volume:surface area ratio decreased.19 The BPO content in the microcapsule was estimated to be about 35 to 40 wt % for the microcapsules prepared at different mixing speeds as determined from iodometery titration method. Since the concentration of BPO was 40 wt % in DBP, these results confirm the calculated overall yield values.


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Table 1. The characteristic properties of Gel-GA/PUF microcapsules at optimal mixing speed and core:shell ratio Mixing

Optimal

Encapsulation

Core

BPO

Mean

Microcapsules’

speed

core:shell ratio

yield (%)

content

content

diameter

shell thickness

(rpm)

(wt %)

(%)

(%)

(µm)

(µm)

500

60:40

65.0 ± 4.2

88.7 ± 2.5

42.6 ± 1.9

198 ±49

1.5 ± 0.16

700

60:40

60.0 ± 3.1

84.6 ± 1.1

39.7 ± 1.5

150 ±31

0.8 ± 0.09

900

60:40

55.4 ± 1.9

81.8 ± 1.0

36.8 ± 2.4

121 ±26

0.67 ± 0.07

Morphology and surface property of the microcapsules after the first (Figure 3) and second (Figure 4) steps were examined by means of optical microscope and SEM. As it is clear in the images, single layer microcapsules had transparent shell, but they exhibited severe stickiness toward each other and were deformed from spherical to hexagonal shape because of the flexibility of shell.

(a)

(b)

Figure 3. (a) Optical microscope image and (b) SEM micrograph of Gel-GA single-walled microcapsules prepared at mixing speed of 700 rpm and core:shell ratio of 60:40 wt %

Wu et al.22 reported similar results for microcapsules with the carboxy-methylcellulose-gelatin wall. In contrast to very smooth morphology of Gel-GA layer (Figure



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3b), roughness of microcapsules was slightly increased after deposition of PUF layer on the first layer (Figure 4b, d and f). Moreover, Gel-GA/PUF microcapsules were welldispersed spherical particles with free flowing characteristic and no sign of stickiness. Furthermore, no deformation was observed under high vacuum in the SEM chamber, and the microcapsules maintained their spherical shape, indicating they had relatively hard shells. SEM study showed that the mean diameter of prepared microcapsules decreased from 200 to 120 µm with increasing mixing speed due to the elevated level of applied shear forces (Table 1).

Mixing speed

Optical image

SEM micrograph

(rpm)

(a)

(b)

(c)

(d)

500

700


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(e)

(f)

900

Figure 4. Optical microscope and SEM images of GEL-GA/PUF microcapsules prepared at constant core:shell ratio (60:40) and mixing speed of (a and b) 500 rpm, (c and d) 700 rpm, and (e and f) 900 rpm.

Shell thickness is a dominant parameter for determination of stability of microcapsules.

A thin shell may result in the early release of active agents and

therefore, reduce the pot life of adhesive. According to Figure 5, the microcapsules had compact and uniform shell structure without any notable defect or hole. Also there was no distinct boundary between Gel-GA and PUF layers indicating that UF pre-polymer likely penetrated into the Gel-GA layer. On the other hand, thickness of the shell decreased from 1.5 to 0.68 µm with increasing mixing speed from 500 to 900 rpm (Table 1). This thickness reduction may be caused by: a) higher turbulence level that inhibits coagulation of coacervate phase on the organic/aqueous interface, and/or b) the ratio of surface area to volume of microcapsules declines with the increase of mixing speed leading to the reduction of shell thickness at a constant core:shell ratio.



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(a)

(b)

(c)

Figure 5. SEM micrographs of ruptured microcapsules shell prepared at a mixing speed of (a) 500 rpm, (b) 700 rpm, and (c) 900 rpm.

FTIR spectroscopy was used for chemical characterization of the synthesized microcapsules (Figure 6). In the FTIR spectrum of shell, the absorption peaks at 1260, 1550, 1650, and 3300 cm-1 correspond to stretching vibration of N-H, stretching vibration of C=O, bending vibration of N-H, and stretching vibration of C-N in Gel and PUF structures, respectively. The existence of stretching bond of C-O-C at 1100 cm-1 confirms the presence of UF in the shell compound. In the FTIR spectrum of the microcapsules, in addition to existence of peaks related to shell, there are peaks at 700, 1750, and 1780 cm-1 that correspond to the out-of-plane bending of aromatic ring, and symmetrical and non-symmetrical stretching vibrations of C=O bonds in BPO. Moreover, appearance of the peak at 1730 cm-1 is also related to C=O stretching in DBP. These results confirm the successful encapsulation of BPO and DBP within the microcapsules.


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1

.1

Shell 1

0

.9

0

.8

0

.7

0

.6

0

.5

0

.4

Microcapsule

4000 3500 3000 2500 2000 1500 1000

500

Transmittance (%)

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0

-1

Wavenumber (cm ) Figure 6. FTIR spectra of microcapsules and shell materials

The particle size distribution plots of the prepared microcapsules are shown in Figure 7. Results show that stirring speed had a significant influence on the average size of the resultant microcapsules. By increasing stirring speed from 500 rpm to 900 rpm, average diameter of microcapsules decreased from 110 to 260 µm. In addition, the size distribution of microcapsules was not significantly affected by variation of mixing speed. Therefore, by controlling the mixing speed during synthesis of microcapsules, the average size of microcapsules can be controlled to fit the desired application.



9

Series1 500 rpm 700 rpm Series1 900 rpm Series1

8

160 µm 125 µm

255 µm

7

Volume Fraction (%)

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6 5 4 3 2 1 0 10

100

1000

Microcapsules diameter (µ µ m) Figure 7. The effect of mixing speed (500, 700 and 900 rpm) on the microcapsule diameter and dm of 60:40 core:shell microcapsules.

DSC was used to examine if BPO maintained its activity after synthesis of microcapsules. Isothermal DSC thermographs of PMM-based resin containing neat BPO (1.5 wt %) and for specimen containing 3.5 wt % microencapsulated samples which contain 1.5 wt % neat BPO (calculated according to BPO content of the microcapsules prepared at mixing speed of 500 rpm in Table 1) are shown in Figure 8. The area under exothermic peak, enthalpy of reaction, was 56.18 mCal/mg and 53.84 mCal/mg for specimen containing 1.5 wt % neat BPO powder and that containing 3.5 wt % BPO-loaded microcapsules, respectively. Exothermal peak appearance as a result of exothermic polymerization of acrylate-based resin indicated that BPO was still active and able to initiate polymerization reaction. However, a slight delay in the curing time and a corresponding decrease in the heat of reaction were observed for microcapsule


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embedded specimen. The decrease in the heat generated from exothermic curing process maybe due to presence of DBP (1.6 wt %) and microcapsule shell (0.39 wt %) in the resin, which are non-reactive materials.

0.40 26.6 min

0.35

Neat BPO -1

mCal.mg 1 ∆Η=56.30 ∆Η=

0.30

Heat flow (W/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


29.2 min

0.25 Microencasulated BPO -1 ∆ H=53.84 mCal.mg

0.20 0.15 0.10 0.05 0.00 0

10

20

30

40

50

60

70

Time (min)

Figure 8. Isothermal DSC thermographs of PMM-based resin containing (a) BPO-loaded microcapsules (3.5 wt %) prepared at mixing speed of 500 rpm, (b) neat BPO powder (1.5 wt %).

Figure 9 shows viscosity variation of MMA-based resin containing crushed microcapsules versus time, measured by rheometer. The graph shows rapid and continuous increase in the viscosity, indicating polymerization of resin. Altogether, it was concluded that BPO activity was not influenced by the microencapsulation process.



1.0E+09 1.0E+08 1.0E+07

Viscosity (Pa.s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1.0E+06 1.0E+05 1.0E+04 1.0E+03 1.0E+02 1.0E+01 1.0E+00 1.0E-01 0

5

10

15

20

25

30

35

Time (min)

Figure 9. Variation of MMA-based resin containing defined amount of crushed microcapsules versus time using rheometer at isothermal mode.

With the aim of examination of microcapsules stability in PMM-based resin, 3.5 wt % of various microcapsules (prepared at mixing speed of 500, 700 and 900 rpm) were dispersed in the resin and viscosity changes were monitored over a 2 months period (Figure 10). The results showed that specimen containing microcapsules prepared at mixing speed of 900 rpm was cured within 3 h. In contrast, resins containing microcapsules prepared at either 500 or 700 rpm showed slight increase in their viscosity due to thicker microcapsule shell. Considering SEM micrographs from crushed microcapsules (Figure 5), instability of the synthesized microcapsules at mixing speed of 900 rpm was likely due to their thin shell thickness (0.67 µm) leading to high permeability of shell to encapsulated materials resulting in premature resin polymerization. However, after 2 months, the microcapsules prepared at mixing speed of 500 rpm were the most stable ones and maintained their spherical shape and BPO particles were observable inside these microcapsules (Figure 10b).


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0.4

(a)

500 RPM 700 RPM

Viscosity (Pa.s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.284

0.3

0.263 0.238

0.241

1 month

2 months

0.221 0.194

0.2 0.149

0.160 0.152

Neat resin

3h

0.1

0 2 weeks

Time

(b)

Figure 10. (a) Change in viscosity of PMM-based resin containing BPO-loaded microcapsules prepared at mixing speeds of 500 rpm and 700 rpm over a 2 months storage period, and (b) optical microscope image of the resin containing microcapsules (3.5 wt %) synthesized at mixing speed of 500 rpm after 2 months.

Considering the fact that toluene is one of the most common solvents used in the acrylate-based resins, the stability of microcapsules in this solvent was studied after being dispersed in toluene for 3 days. As it is apparent in Figure 11, for the microcapsules prepared at 900 rpm, there was no BPO particles inside the microcapsules, and microcapsules deformed and suffered from stickiness after evaporation of toluene.



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Mixing speed

Dispersed microcapsules in toluene

Microcapsules after toluene evaporation

(rpm) 500

700

900

Figure 11. Optical images of microcapsules prepared at different mixing speeds dispersed in toluene after 3 days of immersion (left column) and following by toluene evaporation (right column).

However, BPO particles were observed inside the microcapsules synthesized at 500 and 700 rpm. These microcapsules retained their spherical shape and dispersed well which was the indication of good level of solvent resistance for those microcapsules.


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After 3 days immersion in toluene, stability of the microcapsules prepared at different mixing speeds was investigated using UV analysis and weight loss method. Figure 12 shows UV spectra (a) of ethanol diluted sediment from stability test and measured weight loss (b) from the UV spectroscopy data. The released core materials (BPO and DBP) from the microcapsules prepared at 500, 700, and 900 rpm mixing speeds was 1.5, 9, and 29 wt %, respectively. The results show that with increasing mixing speed, the amount of released core content increase due to reduction of microcapsules shell thickness.

900 rpm

a)

700 rpm 500 rpm

0.4

40

b)

Core released (wt %)

0.5

29

30 20

Absorbance (%)

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9

10

0.3

1.5

0 500

700

900

Mixing speed (rpm)

0.2

0.1

0 200

250

300

350

400

Wavenumber (nm) Figure 12. (a) UV spectra of ethanol diluted sediment from stability test for microcapsules prepared at different mixing speeds and (b) measured weight loss using UV spectroscopy data.



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The reduction of core content after 3 days of immersion in toluene was calculated using weight loss method as 1±0.2, 8.3±1.5 and 27±3.2 wt % for the microcapsules synthesized at 500, 700 and 900 rpm, respectively. The most released core material was observed for the microcapsules prepared at mixing speed of 900 rpm, which was attributed to their thin shell. The gravimetric results are in agreement with their counterpart results obtained from UV spectroscopy.

CONCLUSION In the present work, double-Walled gelatin-gum arabic/polyurea-formaldehyde, GelGA/PUF-based microcapsules containing pre-dispersed benzoyl peroxide (BPO) in dibutyl phthalate (DBP) as core materials were prepared. The microcapsules were synthesized using a two-step process via complex coacervation and in situ polymerization methods at various core:shell ratios and mixing speeds. Optimized preparation conditions were determined for synthesis of free flowing microcapsules. The results showed that BPO was not influenced by the microencapsulation process and maintain its activity. It was further found that Gel improved shell stability toward small organic molecules. However, microcapsules produced at high mixing speeds had a low stability due to small shell thickness. Higher stability of microcapsules prepared at lower mixing speeds toward acrylate-based resin and toluene made them more suitable for application in the acrylate-based adhesives with extended pot life.

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GRAPHICAL ABSTRACT

Schematic representation and SEM micrograph of Gel-GA/PUF double-walled microcapsules

ACS Paragon Plus Environment

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Preparation of Microcapsules Containing Benzoyl Peroxide Initiator

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