Method of producing thermally expansive microcapsule

Details Method of producing thermally expansive microcapsule Method of producing thermally expansive microcapsule

2002-01-17

2003-09-09

Foelak, Morton (Department: 1711)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Cellular products or processes of preparing a cellular...

C521S059000

Reexamination Certificate

active

06617363

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of producing a thermally expansive microcapsule which is particularly excellent both in heat resistance and in solvent resistance and also is extremely low in secondary cohesion when heated.
2. Prior Art
For example, Japanese Patent Publication No, Sho 42(1967)-26524 teaches the method that a thermoplastic polymer is used to micro-encapsulate a volatile expanding agent that comes into its gaseous state at a temperature of equal to or less than a softening point of the thermoplastic polymer, to thereby produce a thermally expansive microcapsule. This publication discloses the method that a foaming agent, such as a low-boiling aliphatic hydrocarbon, is added to a monomer; then, an oil-soluble catalytic agent is incorporated into the monomeric mixture thus obtained; and then, the resultant monomeric mixture is added into a dispersion-agent-containing aqueous dispersion medium with stirring, to be suspended for polymerization, to thereby produce spherical particles including the foaming agent.
However, this method fails to produce the thermally expansive microcapsule having excellent heat resistance and solvent resistance, providing the disadvantage that the micro-capsule is foamed and expanded at low temperature (about 80° C. to about 130° C.) and its expansion ratio is deteriorated at high temperature and long time.
On the other hand, Japanese Patent Publication No. Hei 5(1993)-15499 describes the method of producing a thermally expansive microcapsule wherein a polymer obtained from the component comprising 80-97 weight % of monomer of nitrile series, 20-3 weight % of monomer of non-nitrile series, and 0.1-1 weight % of trifunctional cross-linking agent is used to micro-encapsulate a volatile expanding agent. The thermally expansive microcapsule obtained by this method is superior both in heat resistance in that it is not foamed at temperature of 140° C. or less and in solvent resistance to the conventional one, but it still has the disadvantage of being high in secondary cohesion when heated. Because of this, when the thermally expansive microcapsule is applied, for example, to wallpaper, the wallpaper is indicated as providing a poor aesthetic design. Thus, there is still room for improvement in this method.
SUMMARY OF THE INVENTION
It is the object of the present invention to provide a method of producing a thermally expansive microcapsule that is excellent in heat resistance and solvent resistance, narrow in breadth of the distribution of particle size, and extremely low in secondary cohesion of the particles when heated.
The present invention is directed to a novel method of producing a thermally expansive microcapsule, wherein a polymer obtained from the component comprising not less than 80 weight % of monomer of nitrile series, less than 19.95 weight % of monomer of non-nitrile series, and 0.05-1 weight % of at least tetrafunctional cross-linking agent and/or of long side chain of cross-linking agent is used to micro-encapsulate a volatile expanding agent that comes into its gaseous state at a temperature equal to or less than a softening point of the polymer.
The monomers of nitrile series that may be used in the present invention include, for example, acrylonitrile, methacrylonitrile, á-chloroacrylonitrile, á-ethoxyacrylonitrile, fumaronitrile, or any mixtures thereof. Particularly preferable are acrylonitrile and methacrylonitrile. The amount of monomer of nitrile series used is not less than 80 weight % of the total amount of monomers. The amount of less than 80 weight % of monomer of nitrile series used cannot provide the intended objectives of the solvent resistance and the expandability at high temperature. Further preferably, 90-97 weight % of monomer of nitrile series is used. The monomer of non-nitrile series is selected from the group consisting of acrylic esters including methyl acrylate, ethyl acrylate, butyl acrylate and dicyclopentenyl acrylate, and methacrylic esters including methyl methacrylate, ethyl methacrylate, butyl methacrylate and isobornyl methacrylate. Particularly preferable are methyl methacrylate, ethyl methacrylate and methyl acrylate. The amount of monomer of non-nitrile series used is less than 19.95 weight % or preferably 9.8-3 weight %.
It is a characteristic feature of the present invention that at least tetrafunctional cross-linking agent and/or long side chain of cross-linking agent is used as a cross-linking agent. It is expected that the use of those cross-linking agents provides an improved cross-linking efficiency, so that when heated, the particles can be prevented from becoming sticky all over them, without detriment to their expansibility, to prevent the secondary cohesion of the particles. The at least tetrafunctional cross-linking agents that may be used include, for example, pentaerythritol tetraacrylate, pentaerythritol tetramethacrylate, dipentaerythritol hexaacrylate and dipentaerythritol hexamethacrylate. The long side chain of cross-linking agents that may be used include, for example, polyethylene glycol (PEG #200) diacrylate of number average molecular weight of 200, polyethylene glycol (PEG#400) diacrylate of number average molecular weight of 400, polyethylene glycol (PEG #600) diacrylate of number average molecular weight of 600, neopenthylglycol diacrylate, 1,6-hexanediol diacrylate, 1,9-nonanediol diacrylate, dimethylol-tricyclodecane diacrylate, ethylene oxide modified trimethylolpropane triacrylate, polyethylene glycol (PEG #200) dimethacrylate of number average molecular weight of 200, polyethylene glycol (PEG #400) dimethacrylate of number average molecular weight of 400 and polyethylene glycol (PEG #600) dimethacrylate of number average molecular weight of 600.
For the application to wallpaper, the polymer of the thermally expansive microcapsule according to the present invention may be prepared by further blending a polymerization initiator in the components recited above in an adequate proportion, if desired. No particular limitation is imposed to the polymerization initiator. Although the polymerization initiators that are in common use in the relevant field may be used, an oil-soluble polymerization initiator which is soluble in a polymerizable monomer used is preferably used. For example, dialkyl peroxide, diacyl peroxide, peroxy diester, peroxy dicarbonate and azo compound can be cited. To be more specific, dialkyl peroxides including methyl ethyl peroxide, di-t-butyl-peroxide and dicumyl peroxide; diacyl peroxides including isobutyl peroxide, benzoyl peroxide, 2,4-dichlorobenzoyl peroxide and 3,5,5-trimethylhexanoyl peroxide; peroxy diesters including t-butyl peroxypivalate, t-hexyl peroxypivalate, t-butyl peroxyneodecanoate, t-hexyl peroxyneodecanoate, 1-cyclohexyl-1-methyl ethyl peroxyneodecanoate, 1,1,3,3-tetramethyl butyl peroxyneodecanoate, cumyl peroxyneodecanoate and (á,á-bis-neodecanoyl peroxy) diisopropylbenzene; peroxy dicarbonates including bis (4-t-butyl cychlohexyl) peroxy dicarbonate, di-n-propyl-oxydicarbonate, di-isopropyl peroxy dicarbonate, di (2-ethylethyl peroxy) dicarbonate, dimethoxybutyl peroxy dicarbonate and di (3-methyl-3-methoxybutyl peroxy) dicarbonate; and azo compounds including 2,2′-azobisisobutyronitrile, 2,2′-azobis (4-methoxy-2,4-dimethylvaleronitrile), 2,2′-azobis (2,4-dimethyl valeronitrile) and 1,1′-azobis-(1-cyclohexane carbonitrile) can be cited.
The volatile expanding agent included in the micro-capsule is a material that comes into its gaseous state at a temperature equal to or less than a softening point of the polymer prepared from the combined components recited above. A low-boiling organic solvent is preferably used as the volatile expanding agent. To be more specific, for example, low-molecular-weight hydrocarbons including ethane, ethylene, propane, propene, n-butane, isobutane, butene, isobutene, n-pentane, isopentane, neopentane, n-hexane, heptane and petroleum ether; chlorofluorocarbons including CCl
3
F, CCl

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