Preparation and characterization of thermally expandable microcapsules

by in situ polymerization

Xuebao Wang

Department of Fire Protection Engineering, The Armed Police Forces Academy

Langfang 065000, P. R. China

wxpaul119@163.com

Keywords: thermally expandable microcapsules, in situ polymerization, polyacrylonitrile, polyethyl

acrylate

Abstract. In the present work, we synthesized microcapsules with copolymer of acrylonitrile and

ethyl acrylate as shell and isooctane as core by in situ polymerization, and then optimized the

preparative method. Synthesized microcapsules were characterized by Fourier transform infrared

spectroscopy (FTIR), 13

C nuclear magnetic resonance (13

C-NMR), scanning electron microscopy

(SEM), particle diameter analysis and thermogravimetry (TG).

Introduction

Microencapsulation is a process of enveloping micro-sized materials of solids, liquids or gasses

in a thin hollow microsphere, for the purpose of protecting encapsulated materials from degrading

reagents [1-9]. It has been widely used in the fields of pharmaceutical, biotechnology, agricultural,

intumescent fire retarding, flavor preservation, etc [10-12].

Thermally expandable microcapsules (TEMs) are particles made up of a thermoplastic shell filled

with liquid hydrocarbon. The shell softens and the liquid hydrocarbon boils upon heating. The

hydrocarbon gas works as a blowing agent because the shell expands as the inner pressure increases.

The volume of TEMs expands rapidly to even 100 times [13, 14].

Acrylonitrile is used primarily as a co-monomer in the production of acrylic material. It is a

chemical intermediate in the synthesis of various antioxidants, pharmaceuticals, dyes, and

surface-active agents. In the present work, microcapsules with copolymer of acrylonitrile and ethyl

acrylate as shell and isooctane as core were synthesized by in situ polymerization for the first time

and characterized by Fourier transform infrared spectroscopy (FTIR), 13

C nuclear magnetic

resonance (13

C-NMR), particle diameter analysis, scanning electron microscopy (SEM) and

thermogravimetry (TG).

Experimental

Preparation of thermally expandable microcapsules

Placed distilled water, MgCl2, NaOH, carboxymethyl cellulose (CMC) and NaNO2 into an

oven-dried three-necked round-bottomed flask equipped with a machine stirrer and a condenser,

then added prepared acrylonitrile (ACN), ethyl acrylate (EA), isooctane (ISO) and AIBN mixture.

After stirred for 10 hr at 65 ℃, we adjusted the mixture to proper pH. The mixture was then cooled

to room temperature, filtered, washed with distilled water, and dried. The TEMs powder was finally

obtained.

Fourier Transform Infrared Spectra

The Fourier transform infrared (FTIR) spectra were recorded using a Nicolet 8700

spectrophotometer to identify the chemical structure of the specimens. Samples were mixed with

KBr powders, and the mixture was pressed into a tablet.

Advanced Materials Research Vol. 496 (2012) pp 130-133Online available since 2012/Mar/27 at www.scientific.net© (2012) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.496.130

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13C nuclear magnetic resonance

The 13

C nuclear magnetic resonance (13

C-NMR) measurement was carried out by using a

JNM-AL300 NMR spectrometer with DMSO-d6 as the solvent and TMS the internal standard.

Scanning electron microscopy

The morphology of the TEMs and after gold-sputtered were studies by S-4300FEG scanning

electron microscope (SEM).

Particle Size Distributions Measurement

The mean particle size and distribution of samples were measured by a LA-920 optical particle

sizing instrument.

Thermogravimetric analysis

Thermogravimetric analysis (TGA) was carried out using a TA 2950 thermogravimetric analyzer

at a linear heating rate of 20 ℃ min-1

in air. The weight of sample was kept within 5–10 mg and the

airflow rate was 40ml per minute.

Results and discussion

Characterization of TEMs

The FTIR spectra of TEMs, polyacrylonitrile (PAN) and polyethyl acrylate (PEA) are shown in

Fig. 1 (A). The spectrum of TEMs shows absorption bands at 2245 and 1728 cm−1

. The typical

absorption peak of PAN includes 2245 (C≡N stretching vibration) cm-1

. The typical absorption peak

of PEA includes 1728 (C=O stretching vibration) cm-1

. The spectrum of TEMs reveals that PAN

and PEA exist in TEMs.

Fig.1 (A) The FTIR spectra of TEMs, polyacrylonitrile (PAN) and polyethyl acrylate (PEA). (a)

2245 cm−1

. (b) 1728 cm−1

. (B) 13

C-NMR spectra of TEMs. (C) 13

C-NMR spectra of hollow TEMs

without isooctane.

The 13

C-NMR measurement

13

C-NMR spectra of TEMs and TEMs without isooctane are shown in Fig. 1 (B) and (C). The

characteristic peaks at 173 and 120 ppm confirm the presence of C=O of PEA and C≡N of PAN

respectively. The characteristic peak at 52 ppm confirms the presence of the carbon of CH2 in

isooctane. TEMs have the peaks at 173, 120 and 52 ppm as we can see in Fig. 1 (B). However,

TEMs without core material only have the peaks at 173 and 120 ppm as shown in Fig. 1 (C). These

results indicate the successful synthesis of the TEMs containing PEA and PAN as shell, and

isooctane as core.

Advanced Materials Research Vol. 496 131

Scanning electron microscopy

The surface morphology of TEMs with different mole ratio of ACN/EA, oil/water, shell/core,

different mole of initiator, accelerating agent, polymerization inhibitor and different PH, stirring

speed are shown in Fig. 2. It is clear that the optima preparation conditions of TEMs particles are

ACN/EA 85/15, oil/water 1/3, shell/core 5/1, 1% AIBN as initiator, 5% Mg(OH)2 as dispersing

agent, 0.3% CMC as accelerating agent, 0.3% NaNO2 as polymerization inhibitor, pH 14, stirring

speed 300~350 rpm. It can be seen that the surfaces of TEMs particles are very smooth at above

conditions, but rough or even fractured at other conditions. For example, TEMs with mole ratio of

CAN/EA=85/15 have the most compact shell and round sharp as shown in Fig. 2 (a). With the

increase in content of ACN, the surface of shell becomes less compact even fractured like Fig. 2 (b).

Fig.2 SEM micrographs of outer surfaces of TEMs. (a) TEMs prepared at the optima conditions. (b)

ACN/EA=95/5. (c) Oil/water=1/2. (d) Shell/core=2/1. (e) BPO as initiator. (f) 5% AIBN as initiator.

(g) 3% Mg(OH)2 as dispersing agent. (h) 0.5% CMC as accelerating agent. (i) 0.5% NaNO2 as

polymerization inhibitor. (j) pH 11. (k) Stirring speed 550~600 rpm.

Particle Size Distributions Measurement

The particle size distribution of TEMs is shown in Fig. 3 (A). The size distribution of TEMs is

narrow, and the average diameter of TEMs is 88.13 µm.

Fig.3 (A)The particle size distributions of TEMs. (B) TG curves of TEMs (a) and hollow TEMs

without isooctane (b).

132 Functional Materials and Nanotechnology

Thermogravimetric analysis

Fig. 3 (B) demonstrates the thermal characteristic of TEMs and TEMs without isooctane. It can

be seen that the there are two steps on the degradation of TEMs. At the first step, TEMs decompose

with about 20% weight lost at 120 ℃. At the second step, TEMs begin to decompose at about

330 ℃, which is equal with the curve of TEMs without core material. This result indicates that the

weight lost at 120 ℃ is because of the volatilization of isooctane, which is the core material of

TEMs. The boiling point and flash point of isooctane is 99.3℃ and -12℃ respectively. It is clear

that microencapsulation of isooctane with PAN and PEA shell possesses better heat and

volatilizable gases resistance at high temperature. This makes TEMs suitable for many materials

because of relatively high initial decomposition temperature.

Conclusions

In the present work, we synthesized microcapsules with copolymer of acrylonitrile and ethyl

acrylate as shell and isooctane as core by in situ polymerization, and then optimized the preparative

method. Synthesized microcapsules were characterized by Fourier transform infrared spectroscopy

(FTIR), 13

C nuclear magnetic resonance (13

C-NMR), particle diameter analysis, scanning electron

microscopy (SEM) and thermogravimetry (TG). These results indicate that the polymer of

acrylonitrile and ethyl acrylate encapsulates isooctane very well in TEMs. The size distribution of

TEMs is narrow, and the average diameter of TEMs is 88.13 µm. TEMs have nice heat and

volatilizable gases resistance at high temperature, which makes it suitable for many materials

because of relatively high initial decomposition temperature.

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Advanced Materials Research Vol. 496 133

Functional Materials and Nanotechnology 10.4028/www.scientific.net/AMR.496 Preparation and Characterization of Thermally Expandable Microcapsules by In Situ Polymerization 10.4028/www.scientific.net/AMR.496.130

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