Expandable microspheres and process for producing the same

Details Expandable microspheres and process for producing the same Expandable microspheres and process for producing the same

2000-07-25

2003-09-02

Foelak, Morton (Department: 1714)

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

Synthetic resins

Cellular products or processes of preparing a cellular...

C521S060000

Reexamination Certificate

active

06613810

ABSTRACT:

TECHNICAL FIELD
The present invention relates to foarmable microspheres with a foaming agent enclosed in the shell of a polymer, and more particularly to foamable microspheres extremely sharp in particle diameter distribution. The present invention also relates to a process for producing foamable microspheres extremely sharp in particle diameter distribution by a suspension polymerization process. The present invention further relates to a process for producing foamable microspheres, by which aggregation of polymer particles formed and adhesion of scale to the wall of a polymerization vessel upon polymerization are prevented, and foamable microspheres even in particle shape in the form of a sphere and capable of sharply foaming to provide uniform foams can be provided. The foamable microspheres according to the present invention can be utilized in a wide variety of technical fields including fields of paint and ink.
BACKGROUND ART
In recent years, foamable microspheres have been developed into uses in various fields such as fillers for paints and plastics for the purpose of lightening weight. including the use of foaming ink. The foamable microspheres are generally obtained by microcapsulating a volatile, liquid foaming agent (also referred to as a physical foaming agent or volatile expanding agent) by a thermoplastic resin. Such foamable microspheres have heretofore been produced by a process in which a polymerizable mixture containing at least a foaming agent and a polymerizable monomer is subjected to suspension polymerization in an aqueous dispersion medium. With the progress of the polymerization reaction, a shell is formed by a polymer formed, thereby obtaining the foamable microspheres containing the foaming agent encapsulated in the shell.
In the suspension polymerization process, the polymerizable mixture is generally added to the aqueous dispersion medium containing a dispersion stabilizer, the aqueous dispersion medium containing such a mixture is stirred and mixed to form fine droplets of the polymerizable mixture, and the resultant dispersion is then heated to conduct suspension polymerization. Since the polymerizable mixture forms an oil phase in the aqueous dispersion medium, it can be formed into fine droplets by stirring and mixing. By the suspension polymerization, foamable microspheres having substantially the same particle diameter as these fine droplets are formed. In the step of forming the fine particles of the polymerizable mixture, the stirring and mixing have heretofore been conducted by means of a general agitating blade or a batch-wise high-speed, high-shear type dispersing machine. In the suspension polymerization process, foamable microspheres, the particle shape of which is made even in the form of a sphere, can be provided by suitably selecting a dispersion stabilizer, a polymerization aid and the like. Accordingly, foamable microspheres having properties satisfactory to some extent can be obtained by devising suspension polymerization conditions even when such stirring and mixing method as described above is adopted.
However, when application fields of foamable microspheres are enlarged, and higher performance comes to be required in each application field, the level required of the foamable microspheres is also raised. As the performance of the foamable microspheres, it is particularly important that foaming is sharp, and foams uniform in shape and size can be formed. The phrase “forming is sharp” as used herein means that foam initiating temperatures of individual particles of the foamable microspheres are substantially the same, and the particles initiate foaming all at once under foaming temperature conditions. Therefore, the foamable microspheres are required to have an extremely narrow particle diameter distribution in addition to the even particle shape in the form of a sphere. However, the foamable microspheres obtained in accordance with the conventional process is not sufficiently sharp in particle diameter distribution and hence contain minute particles and coarse particles in plenty based on an average particle diameter. When the particle diameter distribution of the foamable microspheres is broad as described above, foaming conditions among individual particles are delicately different, and so foaming cannot be sharply conducted. In addition, when the particle diameter distribution is broad, foams of uniform size cannot be obtained. Such a tendency is particularly marked when the average particle diameter of the foamable microspheres is great. On the other hand, when classification is conducted to narrow the particle diameter distribution of the foamable microspheres, the process becomes complicated, and a yield is lowered.
The above-described problems are described by specific examples. For example, the use of foamable microspheres broad in particle diameter distribution as a weight-lightening agent or functionality-imparting agent for high-performance paints arises a problem that a finished surface becomes rough due to the presence of coarse particles. The coarse particles are easy to foam at a low temperature to impair sharp foaming. The coarse particles also involves that an expansion ratio cannot be raised because the foaming agent is easy to escape. The minute particles involves a problem that an expansion ratio cannot be raised because the content of the foaming agent therein is low. Such problems become a fatal defect in use of the foamable microspheres for an extremely thin coating film capable of making the best use of the feature thereof.
The foamable microspheres not only are incorporated into ink, paint, plastics and the like in an unformed state, but also may be used in a foamed state according to their uses. More specifically, foams (hollow plastic balloons) of the foamable microspheres are very light-weight and hence come to be used as a filler for paints, for example, for the purpose of lightening the weight of an object to be coated, such as an automobile. Since the foams are very light fine particles generally having a bulk density of about 0.02 to 0.03 g/cm
3
and an average particle diameter of about 20 to 200 &mgr;m, they are easy to escape out in the air when they are taken out of a container and incorporated into a base material for a paint or the like. In addition, the foams gather on the top of the base material upon their stirring and mixing with the base material, and so it is very difficult to uniformly mix them.
Therefore, Japanese Patent Application Laid-Open No. 196813/1995 has proposed a production process of non-scattering foamed microspheres (non-scattering hollow plastic balloons) in which unformed foamable microspheres are mixed with a plasticizer at a temperature lower than the foam initiating temperature of the foamable microspheres, the resultant mixture is brought into contact with another plasticizer heated to a temperature higher than the foam initiating temperature of the foamable microspheres to foam the formable microspheres, and the foamed microspheres are cooled to prevent overfoaming. According to this process, there are merits that {circle around (1)} the unformed foamable microspheres can be dispersed in the plasticizer to make them a fluid state permitting quantitative feeding by a pump, {circle around (2)} uniform non-scattering foamed microspheres can be obtained at the same time as the foaming of the foamable microspheres, and {circle around (3)} the amount of the plasticizer used can be very lessened compared with a process of simply wetting with a plasticizer. In order to adopt such a process, the foamable microspheres are required to have the following properties:
(1) having good solvent resistance because a plasticizer is used as a wetting agent;
(2) exhibiting a viscosity as low as possible when the foamable microspheres are dispersed in the plasticizer in order that the microspheres can be quantitatively fed by a pump;
(3) causing sharp foaming from the viewpoints of process and product quality; and
(4) forming no aggregate upon a foaming process.
Accordingly, there is a

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