Heat-expandable microspheres which have a structure comprising a shell of thermoplastic resin and a blowing agent encapsulated therein are generally called heat-expandable microcapsules. Thermoplastic resins used to make heat expandable microspherers include vinylidene chloride copolymers, acrylonitrile copolymers, and acrylic copolymers; and a blowing agent mainly employed includes hydrocarbons, such as isobutane and isopentane. (Refer to Patent Reference 1.)
Heat-expandable microcapsules having high thermal durability, for example, those comprising a shell of thermoplastic resin produced from a component containing 80 weight percent or more of a nitrile monomer, 20 weight percent or less of a non-nitrile monomer, and a cross-linking agent are disclosed in Patent Reference 2. The method produces heat-expandable microcapsules in a process where a polymerizable mixture comprising a blowing agent, a polymerizable monomer and a polymerization initiator is suspension-polymerized in an aqueous dispersing medium comprising colloidal silica as a dispersion stabilizer (a suspending agent), a diethanol amine-adipic acid condensate as a stabilizing auxiliary, and a polymerization auxiliary.
The polymerization auxiliary is usually used to control the generation of emulsion-polymerization products or prevent the generation of scale in an aqueous medium in the process of suspension-polymerization. Recently, a demand for developing a polymerization auxiliary, which improves the properties of resultant heat-expandable microcapsules and hollow particulates produced by thermally expanding the microcapsules, has been emerged, aside from the purpose for controlling the generation of emulsion-polymerization products and preventing scale in an aqueous medium. Patent Reference 3 discloses a method of producing heat-expandable microcapsules by applying a so-called polymerization inhibitor, such as ascorbic acids and alkali metal nitrites, instead of potassium dichromate. Patent Reference 3 describes that the heat-expandable microcapsules produced in the method sharply expand in heating and are processed into uniformly expanded product (hollow particulates). However, the properties, such as expanding ratio, of the microcapsules are not sufficiently improved. Furthermore ascorbic acids are not preferable because they have poor thermal stability and decompose to lose their function as a polymerization inhibitor during polymerization reaction. In addition, alkali metal nitrites cause a problem, a cost for wastewater treatment after polymerization, because the Ordinance for Water Pollution Control Law which enforces the Water Pollution Control Law of Japan defines the limit values of alkali metal nitrites contained in ground water. Thus ascorbic acids and alkali metal nitrites are not satisfactory as polymerization auxiliaries at present, because they do not sufficiently improve the properties of resultant heat-expandable microcapsules and hollow particulates obtained by thermally expanding the microcapsules, and they cause the problem mentioned above when they are employed in polymerization.
In the conventional research and development for hollow particulates obtained by thermally expanding heat-expandable microcapsules, the variants and ratio of thermoplastic resin and a blowing agent constituting hollow particulates have mainly been studied to improve the properties of the hollow particulates.
Hollow particulates having improved repeated-compression durability are described, for example, in Patent References 4 and 5, where heat-expanded microcapsules having polar groups on their surface are disclosed as hollow particulates which are durable against rupture in mixing and molding a ceramic composition. However, the durability of the microcapsules against rupture in mixing and molding is not sufficiently improved.
Patent Reference 6 describes hollow particulates for lightweight cement articles. The hollow particulates are produced by thermally expanding heat-expandable microcapsules which comprise a shell of polymer obtained from a certain monomer composition and a cross-linking agent, and have an expanding ratio ranging from 20 times to 100 times. However, the durability of the hollow particulates is not sufficiently improved.
Further, Patent Reference 7 discloses a mixture of hollow particulates and heat-expandable microcapsules which are durable against rupture in mixing and molding a ceramic composition. However, the invention has only sharpened the particle size distribution of the mixture, and has not substantially improved the durability.
As mentioned above, hollow particulates having sufficiently improved durability against repeated compression have not been produced, though the durability of hollow particulates against repeated compression has been examined in various ways. The hollow particulates obtained by thermally expanding the heat-expandable microcapsules described in the Patent Reference 3 have also poor durability against repeated compression.
[Patent Reference 1] U.S. Pat. No. 3,615,972
[Patent Reference 2] JP A 62-286534
[Patent Reference 3] JP A 11-209504
[Patent Reference 4] JP A 2003-327482
[Patent Reference 5] JP A 2003-327483
[Patent Reference 6] JP A 2004-131361
[Patent Reference 7] JP A 2005-067943