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Thermally Expandable Microspheres Prepared via Suspension Polymerization - Synthesis, Characterization, and Application
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. (Ytbehandlingsteknik)
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Thermally expandable microspheres are polymeric core/shell particles in which a volatile hydrocarbon is encapsulated by a thermoplastic shell. When these microspheres are heated, they expand and increase their volume dramatically. This volume increase is retained upon cooling, leading to a density reduction from around 1100 kg m-3 to about 30 kg m-3. Since the development in the early 1970´s, microspheres have been used extensively by the industry as a foaming agent or light weight filler.

In this thesis, microspheres with a poly(acrylonitrile-co-methacrylonitrile) shell have been synthesized through free radical suspension polymerization. The microspheres have been characterized with respect to particle morphology and expansion properties in order to deepen the understanding of the microspheres.

It was found that the monomer feed ratio and the polymerization temperature are very important parameters with respect to the expansion properties. Excellent expansion could only be accomplished when polymerizing at 62 °C, with the acrylonitrile feed, fAN, being around 60 mol%, even though core/shell microspheres are formed over a much wider range of fAN. Furthermore, no expansion was achieved when polymerizing at 80 °C, even though no noticeable differences were found, compared to the corresponding sample polymerized at 62 °C.

It was also shown that the expansion properties can be modified by replacing the encapsulated hydrocarbon by another hydrocarbon with a different boiling point. Not only is the boiling point important, the structure of the hydrocarbon is also important. Isooctane which is highly branched was found to give superior expansion compared to linear or cyclic hydrocarbons having a similar boiling point.

Crosslinking of the polymer shell has proven to be very important for the expansion properties. Both the amount and the structure of the crosslinker are important parameters. Especially the maximum expansion can be improved by the crosslinking of the polymer shell. This originates in an increase in the shape persistence of the expanded microspheres at elevated temperatures. By the combination of crosslinkers that are incorporated separately into the polymer shell, the onset temperature of expansion can be increased significantly.

Finally, the surface of microspheres has been modified by grafting poly(glycidyl methacrylate) from the surface by ARGET ATRP. Given that the reaction conditions are appropriate, such modifications can be performed with only limited effects on the expansion properties of the microspheres.

Place, publisher, year, edition, pages
Stockholm: KTH , 2010. , 54 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2010:4
Keyword [en]
Thermally expandable microspheres
National Category
Polymer Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-11981ISBN: 978-91-7415-542-6 (print)OAI: oai:DiVA.org:kth-11981DiVA: diva2:291785
Public defence
2010-02-26, F3, Lindstedtsvägen 26, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Available from: 2010-02-04 Created: 2010-02-03 Last updated: 2010-04-28
List of papers
1. Suspension polymerization of thermally expandable core/shell particles
Open this publication in new window or tab >>Suspension polymerization of thermally expandable core/shell particles
2006 (English)In: Polymer, ISSN 0032-3861, E-ISSN 1873-2291, Vol. 47, no 10, 3315-3324 p.Article in journal (Refereed) Published
Abstract [en]

Acrylonitrile (AN)-methacrylonitrile (MAN) copolymer particles with a core/shell structure were prepared by suspension polymerization. The particles were about 10–20 μm in diameter and had a hollow core containing an inert hydrocarbon. The influence of the monomer feed ratio and the polymerization temperature on the particle morphology was studied. One purpose of this study was to determine the boundaries for achieving a core/shell structure with the polymer encapsulating the hydrocarbon. When polymerizing at 62 °C, it was found that an initial AN/MAN feed ratio (fAN) between 0.15 and 0.9 results in core/shell particles with encapsulated hydrocarbon. fAN lower than 0.15 yielded solid particles with no hydrocarbon encapsulated while fAN higher than 0.9 yielded particles built up entirely from agglomerates of smaller primary particles. In contrast, when polymerizing at 80 °C, a much narrower span of fAN (0.5–0.85) yielded particles with hydrocarbon encapsulated. The influence on monomer conversion and the molecular weight of the polymer was also studied.

Place, publisher, year, edition, pages
Amsterdam: Elsevier Ltd., 2006
Keyword
Suspension polymerization; Core/shell morphology; Encapsulation
National Category
Polymer Chemistry
Identifiers
urn:nbn:se:kth:diva-11975 (URN)10.1016/j.polymer.2006.03.013 (DOI)000237859400006 ()2-s2.0-33646162178 (Scopus ID)
Note
QC 20101001Available from: 2010-02-03 Created: 2010-02-03 Last updated: 2017-12-12Bibliographically approved
2. Thermally Expandable Microspheres with Excellent Expansion Characteristics at High Temperature
Open this publication in new window or tab >>Thermally Expandable Microspheres with Excellent Expansion Characteristics at High Temperature
2010 (English)In: Journal of Applied Polymer Science, ISSN 0021-8995, E-ISSN 1097-4628, Vol. 117, no 1, 384-392 p.Article in journal (Refereed) Published
Abstract [en]

Thermally expandable core/shell particles with a poly(acrylonitrile-co- methacrylonitrile) shell and a hydrocarbon core (blowing agent) have been prepared by suspension polymerization. The objective of this study was to gain a deeper understanding of the parameters determining the expansion properties of these microspheres. It was found that the amount, the boiling point, and the structure of the blowing agent are important parameters for the expansion properties. For example, a higher maximum expansion was reached when using bulkier blowing agents and thus a lower diffusion rate through the polymer shell. Further, the amount and structure of the crosslinker were also found to be essential for the expansion properties. For this particular system, it was found that a dimethacrylate-functional crosslinker gave significantly better expansion when compared with diacrylate-or divinylether-based crosslinkers. Beside these parameters, it was also observed that the particle-size distribution influence the expansion properties of the microspheres.

Keyword
Blowing agents, Coreshell polymers, Crosslinking, Radical polymerization
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-11977 (URN)10.1002/app.31543 (DOI)000277523500047 ()2-s2.0-77950936660 (Scopus ID)
Note
QC 20101022. Uppdaterad från Accepted till Published (20101022).Available from: 2010-02-03 Created: 2010-02-03 Last updated: 2017-12-12Bibliographically approved
3. Influence of Crosslinking on the Characteristics of Thermally Expandable Microspheres Expanding at High Temperature
Open this publication in new window or tab >>Influence of Crosslinking on the Characteristics of Thermally Expandable Microspheres Expanding at High Temperature
2010 (English)In: Journal of Applied Polymer Science, ISSN 0021-8995, E-ISSN 1097-4628, Vol. 118, no 2, 1219-1229 p.Article in journal (Refereed) Published
Abstract [en]

Free-radical suspension polymerization was used to synthesize thermally expandable microspheres (TEMS); in this process, a poly(acrylonitrile-co-methacrylonitrile) shell encapsulated isooctane. Different amounts of dimethacrylate, diacrylate, or divinyl ether functional crosslinker were added to investigate the effects on the crosslinking density of the polymer and the expansion properties of the TEMS. The optimum amount of crosslinker was found to be approximately 0.05-0.1 mol %. However, a significantly better expansion could be obtained with 1,4-butanediol dimethacrylate as a crosslinker, compared to 1,4-butanediol divinyl ether or 1,4-butanediol diacrylate. From monitoring the conversion of monofunctional analogues by gas chromatography, we suggest that the differences in expansion obtained with different crosslinkers, originated from the difference in the reactivity of the radicals in the system toward the vinyl functionalities of the crosslinkers. This regulated the incorporation of the crosslinker into the polymer and, thereby, the mechanical properties of the microsphere shell.

Keyword
blowing agents, core-shell polymers, crosslinking, radical polymerization
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-11979 (URN)10.1002/app.32301 (DOI)000280748400069 ()2-s2.0-77956246293 (Scopus ID)
Note
QC 20101022. Uppdaterad från Submitted till Published (20101022).Available from: 2010-02-03 Created: 2010-02-03 Last updated: 2017-12-12Bibliographically approved
4. Increased Onset Temperature of Expansion in Thermally Expandable Microspheres from Combination of Crosslinking Agents
Open this publication in new window or tab >>Increased Onset Temperature of Expansion in Thermally Expandable Microspheres from Combination of Crosslinking Agents
2011 (English)In: Journal of Applied Polymer Science, ISSN 0021-8995, E-ISSN 1097-4628, Vol. 121, no 1, 369-375 p.Article in journal (Refereed) Published
Abstract [en]

Free-radical suspension polymerization was used to synthesize thermally expandable microspheres (TEMS) in which a poly(acrylonitrile-co-methacrylonitrile) shell encapsulates isooctane. TEMS were prepared with 1,4-butanediol dimethacrylate (BDDMA), in combination with 1,4-butanediol divinyl ether (BDDVE), diallyl carbonate (DAC), or allyl methacrylate, as crosslinkers. It was found that a significant increase in the onset temperature of expansion (T-start) could be obtained when a combination of BDDMA and BDDVE or DAC was used, compared to when only BDDMA was used as crosslinker. The expansion capacity of the TEMS was excellent when BDDMA and DAC were combined. One benefit of the increased T-start was demonstrated when poly(vinyl chloride)-plastisols containing TEMS could be cured at higher temperatures without premature expansion. The differences in expansion obtained with different combinations of crosslinkers originate from the difference in reactivity of the vinyl-functionalities of the crosslinkers, which regulate the incorporation of the crosslinker into the polymer and thereby, the mechanical properties of the microsphere shell.

Keyword
core-shell polymers, blowing agents, crosslinking, radical polymerization
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-11980 (URN)10.1002/app.33585 (DOI)000289353700045 ()2-s2.0-79953705069 (Scopus ID)
Funder
Swedish Research Council, 2003-4573Swedish Research Council, 2004-605
Note
QC 20110211 Uppdaterad från Submitted till Published 20110509Available from: 2010-02-03 Created: 2010-02-03 Last updated: 2017-12-12Bibliographically approved
5. Surface modification of thermally expandable microspheres by grafting poly(glycidyl methacrylate) using ARGET ATRP
Open this publication in new window or tab >>Surface modification of thermally expandable microspheres by grafting poly(glycidyl methacrylate) using ARGET ATRP
2009 (English)In: European Polymer Journal, ISSN 0014-3057, E-ISSN 1873-1945, Vol. 45, no 8, 2374-2382 p.Article in journal (Refereed) Published
Abstract [en]

This study demonstrates the surface modification of thermally expandable core/shell microspheres by grafting glycidyl methacrylate (GMA) using activators regenerated by electron transfer (ARGET) ATRP. To retain the expansion properties it was essential to minimize the shear forces, use solvents compatible with the microspheres and keep the reaction times short (three hours or less). Using microspheres with hydroxyl groups on the surface, it was found that after converting these to α-bromo esters, GMA could be grafted by ARGET ATRP using only 50 ppm of copper catalyst in toluene at 30 °C. Decent control of the polymerization was achieved with PMDETA as ligand reaching PDIs of 1.4 for the solution polymerization of GMA. When microspheres were present, the polymerization was less controlled with higher PDIs. The epoxide groups of the grafted microspheres were hydrolyzed by HCl in THF providing a hydrophilic surface of the microsphere. The expansion property of the microspheres was studied after each reaction step by thermal mechanical analysis, and it was found that the expansion capacity was well preserved with only limited negative effect on the microspheres.

Place, publisher, year, edition, pages
Amsterdam: Elsevier Ltd., 2009
Keyword
Expandable microspheres, Grafting from, Glycidyl methacrylate (GMA), ARGET ATRP
National Category
Polymer Chemistry
Identifiers
urn:nbn:se:kth:diva-11976 (URN)10.1016/j.eurpolymj.2009.05.002 (DOI)000269040700022 ()2-s2.0-67650364304 (Scopus ID)
Note
QC 20100930Available from: 2010-02-03 Created: 2010-02-03 Last updated: 2017-12-12Bibliographically approved

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