Preparation and Properties of Thermoplastic

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Preparation and Properties of Thermoplastic Expandable Microspheres With P(VDC-AN-MMA) Shell by Suspension Polymerization a

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Zhaosheng Hou , Yiran Xia , Wenqiang Qu & Chengyou Kan a

College of Chemistry, Chemical Engineering and Materials Science, Collaborative Innovation Center of Functionalized Probes for Chemical Imaging in Universities of Shandong, Key Laboratory of Molecular and Nano Probes, Ministry of Education, Shandong Provincial Key Laboratory of Clean Production of Fine Chemicals, Shandong Normal University, Jinan, China

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Shandong Provincial Key Laboratory of Biomedical Polymers, Shandong Medical Instruments Institute, Jinan, China c

Department of Chemical Engineering, Tsinghua University, Beijing, China Accepted author version posted online: 10 Dec 2014.Published online: 06 Jan 2015.

To cite this article: Zhaosheng Hou, Yiran Xia, Wenqiang Qu & Chengyou Kan (2015) Preparation and Properties of Thermoplastic Expandable Microspheres With P(VDC-AN-MMA) Shell by Suspension Polymerization, International Journal of Polymeric Materials and Polymeric Biomaterials, 64:8, 427-431, DOI: 10.1080/00914037.2014.958831 To link to this article: http://dx.doi.org/10.1080/00914037.2014.958831

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International Journal of Polymeric Materials and Polymeric Biomaterials, 64: 427–431 Copyright # 2015 Taylor & Francis Group, LLC ISSN: 0091-4037 print/1563-535X online DOI: 10.1080/00914037.2014.958831

Preparation and Properties of Thermoplastic Expandable Microspheres With P(VDC-AN-MMA) Shell by Suspension Polymerization ZHAOSHENG HOU1, YIRAN XIA2, WENQIANG QU1, and CHENGYOU KAN3 1

Downloaded by [Tsinghua University] at 01:57 22 January 2015

College of Chemistry, Chemical Engineering and Materials Science, Collaborative Innovation Center of Functionalized Probes for Chemical Imaging in Universities of Shandong, Key Laboratory of Molecular and Nano Probes, Ministry of Education, Shandong Provincial Key Laboratory of Clean Production of Fine Chemicals, Shandong Normal University, Jinan, China 2 Shandong Provincial Key Laboratory of Biomedical Polymers, Shandong Medical Instruments Institute, Jinan, China 3 Department of Chemical Engineering, Tsinghua University, Beijing, China Received 10 April 2014, Accepted 24 August 2014

Thermoplastic expandable microspheres (TEMs) having core=shell structure were prepared via suspension polymerization with vinylidene chloride (VDC), acrylonitrile (AN), and methyl methacrylate (MMA) as monomers and i-butane as blowing agent. TEMs were about 20 mm in diameter and had a hollow core containing i-butane. The influence of the monomer feed ratio and blowing agent content was researched. When the monomers composition of 58.4 wt% VDC, 28 wt% AN, 13.6 wt% MMA, and 32 wt% i-butane in oil phase, suspension polymerization could yield TEMs having good expansion properties. The maximum expansion volume was 25 times of original volume at about 111–120 C, the blowing agent content in microspheres was about 21.5 wt%. The Tm.e, To.e, and To.s. of the TEMs increased with the VDC content in the polymerizable monomers decreasing.

Keywords: Microspheres, properties, suspension polymerization, thermal expansion

1. Introduction Address correspondence to: Zhaosheng Hou, College of Chemistry, Chemical Engineering and Materials Science, Shandong Normal University, Jinan 250014, China. E-mail: [email protected] Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gpom.

The thermoplastic expandable microspheres (TEMs) consist of a drop of liquid hydrocarbon, used as the blowing agent, is contained in a 3 7 mm-thick thermoplastic polymer shell [1]. Before expansion, the typical diameter is about 20 mm. When TEMs are heated above the glass-transition temperature (Tg)


428 of the polymer shell, the diameter increases to 80–100 mm and the density drops from 1100 kg=m3 to approximately 20–30 kg=m3 [2]. The volume of the particles is retained upon cooling because of the plastic deformation of the polymer shell, and hence, the expansion is not reversible. The TEMs have proven to be a very useful product in a wide variety of applications as blowing agents or as lightweight fillers, and in printing inks to get three-dimensional patterns on paper, wallpaper, and textiles [3–6]. Microspheres’ surface modifications with functional groups give TEMs more applications [7–9]. Interesting new applications of microspheres are as single-use pumps or valves in microfluidic systems [10,11]. The TEMs are prepared by suspension-type polymerization of droplets of a mixture of monomer and blowing agent. Their manufacturing method was first described by the Dow Chemical Co.’s patent in 1970s [1], and since then there have been many other patents [12–15]. Even though TEMs have been commercially available for nearly 30 years, there are only a limited number of literatures published regarding the preparation and properties of TEMs [16–20]. During heat exposure, the ductility and intensity of the shell are dominant factors, which are closely related with the properties of TEMs. In most studies [2,17,18], acrylonitrile (AN) and=or methacrylonitrile (MAN) with gas-barrier property are the most commonly used as polymerizable monomers. However, AN is a highly toxic substance, and the toxicity of MAN is lower than AN, but MAN is more expensive. So, using lower toxic and cheap monomers to partly or completely substitute for nitrilecontaining monomers to prepare TEMs is the objective of the study [19]. In this study, the TEMs were prepared by suspension polymerization with AN, methyl methacrylate (MMA), and vinylidene chloride (VDC) as monomers, i-butane as blowing agent. The effects of monomer composition and blowing agent content on the thermal expansion properties are researched.

2. Experimental 2.1 Materials VDC (99%, Sigma, USA) was used without further purification. AN and MMA (AR, Beijing Chemical Reagent Co., Ltd, Beijing, China) were distilled before use. 2,20 Azobisisobutyronitrile (AIBN, AR, Beijing Chemical Reagent Co., Ltd, Beijing, China) used as initiator was recrystallized in ethanol three times and dried under vacuum at room temperature. Ethoxylated trimethylolpane triacrylate (ETMTA; Industrial grade, Shanghai Eternal Techno. Co. Ltd, Shanghai, China) was used as cross-linking agent and i-butane (Industrial grade, Jiangshu Sinopec Yangzi Petrochemical Co. Ltd, Nanjing, China) was used as blowing agent. Colloidal silica (10 12 nm diameter, pH ¼ 9 10, 30% solid content), Poly(vinyl pyrrolidone) (PVP; Mw ¼ 40,000 g=mol), and other reactants and solvents were purchased from Beijing Chemical Reagent Co., Ltd (Beijing, China) and was used as received.

Z. Hou et al. 2.2 Instruments and Characterization First, the polymerization was carried out in an autoclave (1.0 L, Jiangshu Jiangyan Universal Instrument Factory, China) equipped with a stacked impeller (lower anchor impeller and upper Rushton turbine impeller). And the blowing agent was added to the autoclave with a pressure addition burette (0.25 L, Jiangshu Jiangyan Universal Instrument Factory, China) under pressure provided by nitrogen bottle. Second, a polarizing optical microscope (POM) was employed to study the microsphere morphologies. The unexpanded and expanded microspheres were fully dispersed in deionized water, and a droplet of the suspension was put between a slide and a cover glass. The morphologies were obtained with a POM (Nikon104, Nikon Corporation, Japan) equipped with Panasonic wv-CP240=G camera system. Third, particle size and size distribution was determined on a Malvern Mastersize Hydro 2000 SM dynamic lightscattering apparatus (United Kingdom). Distilled water was used as dispersing medium and the dilute water dispersions of the microspheres were slightly sonicated for 10 min to break any agglomerates. Fourth, thermogravimetric analysis (TGA) was conducted on a TGA 2050 (Universal, USA) to determine the blowing agent loss and any residual monomers in the microspheres. The sample was heated from 50 to 450 C at 10 C=min under N2 atmosphere. Fifth, the density of unexpanded and expanded microspheres was measured with a graduated test tube. The sample was added to the tube which was weighed as m1 (g), and the tube was gently vibrated, then the mass of the tube containing sample and the volume of the sample was recorded as m2 (g) and V1 (mL), respectively. The value of (m2 – m1)=V1 was the density of unexpanded microspheres (g=mL). The tubecontaining sample was heated up to the sample expanding to the maximum volume, and the maximum volume and the mass of the tube-containing sample was recorded as V2 and m3, respectively. The value of (m3 – m1)=V2 was the density of expanded microspheres (g=mL). Obviously, the value of V2=V1 was the maximum volume expansion ratio (Vmr). Last, the temperature at maximum expansion volume (Tm.e.), the temperature onsets of expansion (To.e.) and shrinkage (To.s.) were measured with an oil bath. To a test tube was added 0.2–0.3 g sample, and the tube was then placed into an oil bath which was heated from 70 to 150 C at a rate of 5 C=min. The changes of the sample morphology were carefully observed and the To.e, Tm.e., and To.s. were recorded. 2.3 Preparation of TEMs Preparation of TEMs was preformed according to a process of suspension polymerization which included five steps described as follows (taking Sample 4 as example), and the process to prepare TEMs is illustrated in Figure 1. 1. Water phase: Sodium chloride (4.24 g), sodium nitrite (3.56 g, 2.5 wt% aqueous solution), colloidal silica (19.5 g, 30% solids), and PVP (3.0 g, aqueous solution, 20 wt%) were dissolved in deionized water (210.0 g). After

Thermoplastic Expandable Microspheres With P(VDC-AN-MMA) Shell


Fig. 1. Schematic diagram for preparing TEMs.

the aqueous phase was mixed uniformly, the pH of the mixture was adjusted to about 3.5 with hydrochloric acid (2.0 mol=L), and then the water phase was cooled to about 5 C. 2. Oil phase: EOTMTA (0.95 g, 0.5 wt% of oil phase) and AIBN (1.40 g, 1.0 wt% of monomers) were dissolved into the monomer mixture containing AN (39.4 g), VDC (82.2 g) and MMA (19.1 g), and the mixture to achieve uniformity. Then the oil phase was cooled to about 5 C. 3. Homogenization and polymerization process: The aqueous phase and oil phase were transferred into the autoclave and the autoclave was sealed. The system was purged with N2 for about 3 min to exclude the oxygen, and then the autoclave was resealed and pressurized with N2 to about 34 bars. The pressure addition burette containing i-butane (54.2 g, 27.5 wt% of oil phase) was connected with the autoclave, and the i-butane was added into the autoclave under pressure provided by nitrogen bottle. Once the i-butane was completely added, the mixture was sheared at 12001300 rpm for about 12 min. Then, the stir speed was dropped to 230250 rpm and the temperature was raised to 65 C with water bath to initiate suspension polymerization reaction. 4. Work-up: After polymerization for 20 h, the autoclave was cooled down to room temperature by ice water, and the pressure decreased slowly to normal pressure. Agglomerates and larger particles were removed from the slurry using a 100 mm sieve. The products were obtained using a suction filtration assembly with qualitative filter paper, and dried for more than 48 h at ambient temperature and pressure after thorough washing with water. The other samples were prepared with the same process except monomer composition and blowing agent content.

3. Results and Discussion 3.1 Homogenization Velocity and Time To obtain the microspheres with diameter of about 20 mm, the proper homogenization velocity and time should be obtained before TEMs prepared. The oil phase and aqueous phase were got according to the recipes of Sample 4 besides using n-hexane with same volume to substitute for i-butane. Because the boiling point of n-hexane was 69 C, the homogenization process could be carried out at normal pressure. The optical micrographs of the mixture of oil phase and aqueous phase, which was homogenized with the stacked impeller in autoclave at 1300 RPM for 6 min and 12 min and cured for 5 min, are shown in Figure 2. There were many droplets which diameter was much larger than 20 mm in Figure 2a, while all the most of droplets in Figure 2b had a

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1520 mm diameter, which satisfied the requirement. The homogenized suspension was poured to a tube that was sealed and placed at normal temperature and pressure, and it had no change after 6 h, which showed the suspension having better stability. So, to obtain microspheres with satisfied diameter, the optimal homogenization velocity and time were 1300 RPM and 12 min, respectively. 3.2 Micromorphologies of Sample 4 The micromorphologies of microspheres were observed with POM before and after expanded. The optical micrographs of Sample 4 are shown in Figure 3. The diameter of the unexpanded microspheres (Figure 3a) was about 20 mm, and it could be observed clearly that there was a small liquid oscillating drop centered in every microsphere. Apparently, the small liquid drop was blowing agent (i-butane). The diameter of the expanded hollow microspheres was about 4080 mm (Figure 3b), the uneven size could be due to the wider particle size distribution of unexpanded TEMs and the different content of blowing agent in microsphere. How to prepare TEMs with narrow size distribute was a difficulty of suspension polymerization, which needed further studies. 3.3 Thermal Stability of Sample 4 Figure 4 was the TGA curve of Sample 4. The weight loss of the microspheres at 85–105 C was about 11% (Figure 4, I), which was due to the loss of aqueous and little of residual monomer in the microspheres. The weight loss at 115–170 C was about 19% (Figure 4, II) which should be due to the loss of blowing agent encapsulated in the microspheres. When the temperature was higher than the Tg of the microspheres, the blowing agent would be leaked out from the softening polymer shell. From the results of TGA, it could be determined that the blowing agent content in microspheres was about 19 wt%, which was lower than that in raw materials. The weight loss above 250 C attributed to the decomposition of copolymer, obviously. 3.4 The Effect of the Monomer Composition and Blowing Agent Content on the Expansion Properties A series of TEMs with P(VDC-AN-MMA) shell were prepared using different monomer composition as in Table 1, and expansion properties of the samples were shown in Table 2. The Vmr was mainly affected by the monomer of AN. When the content of AN in monomers was lower than 28 wt%, the TEMs had unsatisfied Vmr. (Samples 2, 3). The Tm.e, To.e, and To.s. of the TEMs (Samples 25) increased with the VDC content in the polymerizable monomers decreasing [21], which should be due to the low Tg (19 C) of PVDC, while PAN and PMMA were both rigid structure having Tg of 97 and 105 C, respectively. To prepare the TEMs, VDC and MMA could be used to replace part of nitrile-containing monomer, and Tm.e, To.e, and To.s. could be controlled by changing the ratio of VDC and MMA, but the content of MMA in monomers should not be higher

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Fig. 2. Optical micrographs of the mixture homogenized at 1300 RPM for (a) 6 min and (b) 12 min.

Fig. 3. The polarized micrographs of (a) unexpanded and (b) expanded Sample 4.

than 13.6 wt% (Sample 5). The Tm.e and To.s. of Expancel (Sample 642 WU40) were higher than those of samples prepared in this work, it could attribute to high crosslinking density and=or more nitrile-containing monomer used in shell of Expancel. When the content of i-butane (blowing agent) in oil phase increased from 27.5 to 32 wt%, the Vmr of the corresponding samples (Samples 4, 6, 7) increased from 20 to 25, which was higher than that of Expancel. While the content of i-butane

increased to 34 wt% (Sample 8), the Vmr decreased to 16. The reason may be that the shell of expanded microspheres was too thin when using higher blowing agent content, and the microspheres easily ruptured during expansion process. So, the optimum content of i-butane in oil phase was about 32 wt%. 3.5 Particle Size and Size Distribution The average particle size of the TEMs was 15–20 mm (Table 2), which showed that the homogenization conditions were suitable. But the microspheres had wider size distribution, which was the common phenomenon of suspension polymerization. According to the research results of Torza and Mason [22], it was only possible to form a complete microcapsule when the interfacial tension between shell copolymer and water phase was smaller than that between core material and water phase. There were papers reported to use surfactant [18] or low mass molecule alcohol [23] in water phase to prepare TEMs, the results showed that the size distribution was only slightly decreased. Table 1. Monomer composition in preparation of P(VDC-ANMMA) TEMs Sample

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VDC (wt%) 72.0 72.0 66.7 58.4 56.5 58.4 58.4 58.4 AN (wt%) 28.0 22.7 22.7 28.0 28 28 28 28 MMA (wt%) 0 5.3 10.6 13.6 15.5 13.6 13.6 13.6 Fig. 4. TGA curve of Sample 4.

Monomer content in monomers.

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Thermoplastic Expandable Microspheres With P(VDC-AN-MMA) Shell Table 2. The properties of P(VDC-AN-MMA) TEMs Sample

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i-Butane in oil phase (wt%) 27.5 27.5 27.5 27.5 27.5 30.0 32.0 34.0 i-Butane in TEMsb (wt%) 20.0 — — 19.0 18.5 20.5 21.5 22 Average particles diameter (mm) 18.3 18.1 17.4 18.4 20.2 19.6 17.9 19.1 Vmr 22 13 14 20 17 22 25 16 Tm.e ( C) 115–125 114–120 112–120 110–116 107–114 111–120 111–120 110–116 94 90 87 85 85 86 86 86 To.e. ( C) To.s. ( C) 130 127 125 125 121 126 127 121 Density before expanded (g=mL) 0.46 0.43 0.59 0.41 0.42 0.45 0.46 0.44 Density after expanded (kg=m3) 19.0 28 28.5 19.3 18.0 19.8 15.2 26.2

642WU40a — — 16c 24 120–128 85 140 — 17c

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Expancel sample. Data obtained from TGA. Data obtained from Expancel.

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size distribution, which needs further studies. The Tm.e, To.e, and To.s. of the TEMs could be controlled by changing the VDC content in the polymerizable monomers.

Funding This work was supported by Shandong Provincial Natural Science Foundation, China (No. ZR2013EMM004).

References

Fig. 5. Particle size distribution of TEMs prepared with ethanol (Sample 4’) or without ethanol (Sample 4).

According to the same recipes and preparation process of Sample 4, Sample 4’ was prepared except adding 1 wt% ethanol in water phase. The particle size and particle size distribution of Samples 4 and 4’ was showed in Figure 5. It could be seen that the TEMs prepared with ethanol had a smaller average size and a slightly narrower size distribution than those prepared without ethanol, but it was also dissatisfied. How to prepare TEMs with narrow size distribution needs further studies.

4. Conclusions TEMs having P(VDC-AN-MMA) copolymers as shell and i-butane as blowing agent, were prepared via suspension polymerization. When there was monomer composition of 58.4 wt% VDC, 28 wt% AN, 13.6 wt% MMA, and 32 wt% i-butane in oil phase, suspension polymerization could yield TEMs (Sample 7) having good expansion properties. The maximum expansion volume was 25 times of original volume at about 111–120 C, the blowing agent content in microspheres was about 21.5 wt%. The TEMs prepared in the study had average particle size of about 20 mm and wide particle

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