Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Particulate matter
Reexamination Certificate
2000-08-24
2001-05-22
Acquah, Samuel A. (Department: 1711)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Particulate matter
C428S402000, C428S402200, C428S402220, C524S457000, C524S849000, C524S879000, C106S014350, C106S014360, C106S014420
Reexamination Certificate
active
06235394
ABSTRACT:
TECHNICAL FIELD
The present invention relates to heat-expandable microcapsules having superior heat-resistance, the process producing the same and the application thereof.
TECHNICAL BACKGROUND
Heat-expandable microcapsules comprising thermoplastic polymer shell in which low-boiling-point solvent is encapsulated have been under investigation for a long time. For example, the technique disclosed in Japanese Patent Publication Sho-42-26524 relates to a general production process of heat-expandable microcapsules. The technique disclosed in Japanese Patent Laid Open Sho-62-286534 and Japanese Patent Laid Open Hei-5-285376 (U.S. Pat. No. 5,536,756) provides the processes for producing heat-resistant heat-expandable microcapsules wherein the shell of heat-expandable microcapsules is formed by polymerizing the monomers containing 80% or more of acrylonitrile or the monomers which form homopolymers of high glass-transition point for the purpose of increasing the starting temperature of microcapsules' expansion and by adding a small amount of radically polymerizing polyfunctional monomers to the above monomers for the purpose of improving the heat-resistance of resultant microcapsules with the polyfunctional monomers which thermally polymerize and harden the shell of the microcapsules at their expansion. Those processes do not work well as expected if the polymers are not cross-linked spontaneously and densely when the microcapsules are heated.
Recently the character of synthetic polymers has been greatly changed since engineering plastics were developed and Kaminsky's catalyst was invented. In addition, the needs for recycling and reuse of polymers have emerged with the change of the social concept toward environmental problems. And crystalline polymers such as polyethylene, polypropylene and PET are used frequently instead of cross-linking polymers such as polyurethane.
Under such situation, new expanding agents, which expand at higher temperature than conventional ones, are required. In addition, they are required to have heat-resistance durable enough during long-time use at high temperature. Organic expanding agents meet those requirements but they are apt to form a continuous cell, which has poor property such as low tenacity of polymer. The inventors have produced microcapsules that are durable under high-temperature molding and expand to produce single closed cells when required.
The microcapsules can also be applied to various fields as a filler of low density and high heat resistance when the microcapsules are expanded and processed into composite with inorganic components for controlling their specific gravity.
The expanding temperature of conventional expandable microcapsules having polymer shell of which main component is acryl ester depends on the Tg and molecular weight of the polymer and on the boiling point of encapsulated liquid. For increasing the starting temperature of microcapsules' expansion, a liquid of higher molecular weight needs to be encapsulated and thus the quantity of the encapsulated liquid must be increased to keep the internal pressure of vaporized liquid, which decreases with the increase of molecular weight of an encapsulated liquid, at a proper level. In this case, the shell of microcapsules becomes thinner due to the decrease of the ratio of polymer that forms the shell of microcapsules. At the initial step of expansion, such thin shell cannot retain encapsulated liquid (gas of vaporized encapsulated liquid) in microcapsules against the high pressure of the vaporized liquid and the gas is exhausted drastically to decrease the expansion ratio. In addition, the maximum expansion ratio is apt to be lowered.
Drastically softened polymer shell cannot retain gas in microcapsules and results in the leakage of gas through shell. Polymer shell of higher softening point (or higher Tg) is desirable for microcapsules durable at high temperature.
Many of the acryl ester polymers having high Tg have bulky structure. And the polymers of which molecular chains are not thermally kinetic due to steric hindrance have high Tg. Thus their molecules have wide interspace and do not soften easily owing to their bulky structure.
Such property is preferable to the softening of polymer shell that results in the dissolution and release of the molecules of encapsulated liquid, but it is disadvantageous for producing heat-resistant microcapsules (which retain the gas of encapsulated liquid within polymer shell).
DISCLOSURE OF INVENTION
Considering the result, we have screened out monomers satisfying the following two requirements.
(1) Making homopolymer of high Tg
(2) Comparatively small molecules, which are apt to crystallize
Only a few monomers meet the requirements and crystalline monomers are highly soluble in water. Thus some modification is necessary for producing microcapsules from the monomers as main components satisfying the above requirements because they cannot be processed into microcapsules in the ordinary reaction procedure.
The inventors paid their attention to the high Tg, heat-resistance, high hydrogen-bonding tendency and superior gas-diffusion preventability of polycarboxylic acid monomers, and investigated on the parameters for producing microcapsules from such monomers. Polycarboxylic acids are stable and insoluble in water in acidic form. Thus their derivatives were found to function well as the shell of microcapsules so far as they are kept in acidic or neutral. In addition, their molecules easily associate owing to their high hydrogen-bonding tendency, rarely diffuse encapsulated solvent and well retain the gas in microcapsules.
Acrylic acid monomers cannot be processed into microcapsules of satisfactory quality in an ordinary production process due to their high solubility in water. Because acrylic acid monomers cannot be retained in the oil phase of the dispersion system in an ordinary production process of microcapsules wherein monomers are dispersed in water and processed into polymer shell through in situ polymerization from oil phase
For dissolving acrylic acid in oil phase, highly water-soluble substances, such as inorganic salts, are added in water phase to disperse acrylic acid into oil phase forcibly with salting-out effect. And thus acrylic acids gather nearer to the interface to form a shell having acrylic-acid-rich outer layer. And the degree of crystallization of the polymer shell reaches to a sufficient level owing to the association of carboxylic acid and the shell functions satisfactorily as heat-expandable microcapsules (foaming microcapsules).
With the monomers satisfying the above two requirements, heat-expandable microcapsules comprising a polymer of high Tg and short intermolecular distance and having stable expansion performance to keep encapsulated liquid stably at high-temperature foaming region, in other words, having so-called high-temperature foaming performance have been invented.
However, the microcapsules designed on the basis of such performance, of which main components are acrylonitrile and acrylic acid monomers, had insufficient performance for foaming stably within a wide temperature range, though they had improved expansion behavior at high temperature. Because the resin was thermoplastic and not durable against heat to soften drastically at high temperature, to fail to keep the capsule structure and to facilitate the diffusion of encapsulated gas through polymer shell, though the resin was imparted with elasticity by cross-linked molecules.
Then the inventors improved the heat resistance of the microcapsules with the following design. Cross-linking and hardening the polymer of the outer shell after the expansion of microcapsules can improve the heat-resistance of microcapsules. And high-density three-dimensional cross-linkage makes shell resistant enough against shrinkage even when the shell is very thin and improves the heat-resistance of microcapsules drastically.
The monomers having functional groups, which cross-link with carboxylic acids, are monomers containing methylol, epoxy, amino or hydroxy
Shimazawa Toshiyuki
Takahara Ichiro
Acquah Samuel A.
Foley & Lardner
Matsumoto Yushi-Seiyaku Co. Ltd.
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