JP3602290B2 - Gypsum-based investment for high-temperature dental casting - Google Patents

Description

【０００８】
このうち石膏系の結合材を用いる石膏系埋没材の場合には、操作性（流動性）や堀出性がよく、残留応力による変形や経時変化がないなどの諸点では優れている。しかし耐熱性に劣り、特に鋳造温度が高い場合には石膏の熱分解によりガスが発生し、これが焼き付けや鋳巣の原因となり、目的とする鋳造体が脆化し、或いは汚染されるなど、どうしても越えられないマイナス面の特性をもつという問題があった。石膏は、例えば、その種類により異なるが温度８００℃若しくは８５０℃程度から分解が起こり、特に減水剤等の存在によりその石膏系複合体に還元が生じることにより石膏の分解が促進されることが知られている。
【０００９】
このため、例えばロストワックス法に適用し得る埋没材としては、金合金、銀合金などの比較的溶融点の低い合金（すなわち低溶合金）の場合には石膏系の埋没材が使用され、陶材焼付用の貴金属系合金（プレシャス系＝ＰＲ系）や陶材焼付用のセミプレシャス系合金、或いはＮｉーＣｒ合金などの比較的溶融点の高い合金（すなわち高溶合金）の場合には、リン酸塩系の結合材を用いるリン酸塩系埋没材が使用されている。
【００１０】
上記のように高溶合金の鋳型材としてリン酸塩系埋没材が使用されている理由は、表１に示すように高温鋳造特性すなわち高温鋳造時にガス発生がなく、また高溶合金でみられる大きな鋳造収縮をリン酸塩系埋没材の凝結時の大きな膨張で補償することができるためである。しかし、このリン酸塩系埋没材は使用時に粘性の高いコロイドシリカ溶液を使用することから操作性に難があり、また凝結膨張が不均一であるので、残留応力によりワックスパターンが変形することや鋳型強度が大き過ぎることにより、掘り出し時に目的とする鋳造体を破損するなどの欠点を有している。
【００１１】
一方、従来の石膏系埋没材の場合には、この埋没材で作製した鋳型は適当な強度を持っているので鋳造体を破損することなく掘り出すことができるが、高溶合金の溶湯が石膏と接触すると石膏が熱分解してガスが発生し、焼き付けや鋳巣の原因となるなど根本的に越えられない物性を持つという欠点がある。このため、この課題を克服する手法として特開昭５０ー１１３４１７号公報や特開昭６１ー２０５４７号公報、或いは特公昭５３ー１９８７７号公報などこれまで各種工夫がなされている。
【００１２】
しかし、これら技術における鋳造温度は何れも高々７５０℃以下であり（温度７５０℃まで触れたものは唯一特開昭５０ー１１３４１７号公報の場合のみ）、また埋没材の構成成分としてクリストバライトや石英、或いは珪石などのＳｉＯ_２ 成分を使用したものであるため、これを例えばプレシャス合金のような高溶合金の鋳造用として適用する場合、鋳造金属とＳｉＯ_２ 系成分との反応、石膏成分の分解反応によるガスの発生等により、製品である鋳造物に肌荒れが生じるなどの問題がある。しかもこの場合、粒径５０μｍ以上のＳｉＯ_２ 系成分を用いた場合には、その粗粒成分により鋳物の表面状態が著しく悪くなるだけでなく、鋳造体の寸法が小さくなり、適合性が不良となるという問題がある。
【００１３】
さらに特開昭６３ー１４１９０６号公報の埋没材組成物においては、耐火材としてアルミナ等を使用し、膨張剤として天然でんぷん及び周期律表第ＩＶ、Ｖ、ＶＩ族の遷移金属の炭化物、窒化物、硼化物、珪化物、硫化物の一種又は二種以上を添加することにより、膨張が緩やかで冷却時の収縮が小さくなり、合金の鋳造収縮の補償がさらに改善されたと説明されているが、しかしこの場合にも、半水石膏を結合材として使用すると、鋳造時に石膏の熱分解によりガスが発生し、これにより鋳物の鋳造不良が生じる等の問題がある点に変わりはない。
【００１４】
このように、これら何れにしても、高温で適用される高融点金属の鋳造時において、石膏それ自体が備える良好な操作性に加え、石膏系埋没材に要求される鋳造性、表面粗さ、掘り出し性、鋳造金属の寸法適合性等をも満足した技術は開示されていない。この点、本発明者等は、石膏系埋没材に対してコバルト化合物、チタン化合物、タングステン化合物、ニッケル化合物、クロム化合物、マンガン化合物、亜鉛化合物、セレン化合物、或いはスズ化合物を添加することにより、金属鋳造時に生じる石膏の分解を抑制し、鋳造不良を防止できる石膏系埋没材組成物を開発しており（特願平８ー２９１２３号）、これにより鋳造体における寸法変化に関する適合性を満足させることができる。
【００１５】
ところが、それを満足させるためにはそれら成分の添加量を多くする必要があるが、その添加量を多くすると鋳造体の表面が荒れてしまうという不都合も生じる。そこで、さらに本発明者等は、石膏系鋳造埋没材の耐熱材成分としてマグネシア／アルミナ系スピネルに着目し、これを耐熱材として用いることにより、石膏の分解を抑制するとともに表面状態を改善し、操作性や堀出性が良好で、変形や経時変化がないなどの石膏系埋没材本来の特性を保持しつつ、高温鋳造特性すなわち熱膨張の問題を無くし、耐高温分解性等を改善している（特願平８ー２６９３５９号）。
【００１６】
ところで、鋳造時において、残存ワックス等の炭素の影響により、主として次式（１）から（２）の反応が起こり、式（３）の分解方式をとることが知られている。このため、特に温度１４００℃以上で鋳造され、鋳物の埋没材中での高温保持時間が長い場合には、その石膏の熱分解を抑制することが不可能となり、石膏の熱分解によるガス発生により鋳造不良を引き起こした。
【化 １】
ＣａＳＯ_４ ＋ ４Ｃ → ＣａＳ ＋ ４ＣＯ ↑ （１）
３ＣａＳＯ_４ ＋ ＣａＳ → ４ＣａＯ ＋ ４ＳＯ_２↑ （２）
４ＣａＳＯ_４ ＋ ４Ｃ → ４ＣａＯ ＋ ４ＣＯ↑ ＋ ４ＳＯ_２↑ （３）
【００１７】
上記鋳造不良をなくする別の方法として、埋没材中の通気性（気孔率）を向上させ、ガス抜けを良好にする方法が考えられる。このため使用するフィラーの粒度を考慮したものとして特公昭５２ー１１９２１号公報や特開昭５４ー１０４６９４号公報がある。このうち特公昭５２ー１１９２１号公報の歯科金属床鋳造用石膏埋没材では、珪石粉末５０〜６０％、超硬質半水石膏粉末４０〜５０％、可溶性塩化物粉末１〜２％よりなる粉体混合物に水を練和してスラリーとするが、該珪石粉末について、粒度分布として１００〜２００メッシュ１０％以下、２００〜３２５メッシュ１５％以上３２５メッシュ篩を通過するもの７５％以下なる粒度に粉砕したものが用いられる。
【００１８】
また、特開昭５４ー１０４６９４号公報においては上記三成分珪石粉末、超硬質半水石膏粉末及び可溶性塩化物粉末の量的割合の範囲について幾分異なるが、粒度分布として２００〜３２５メッシュ２５〜４０重量％、３２５メッシュ篩を通過するもの６０〜７５重量％なる珪石粉末を使用し、超硬質半水石膏粉末についても珪石粉末と同じ粒度分布とするというものである。しかし、これらによっても高温鋳造性を改善するまでには至っていない。
【００１９】
【発明が解決しようとする課題】
本発明は、耐熱材と結合材としての半水石膏を含む歯科用高温鋳造用石膏系埋没材において、これに加える該石膏の減水剤として６００℃以下で燃え抜ける減水剤を使用することにより、高温鋳造性を改善してなる耐高温分解性に優れた歯科用高温鋳造用石膏系埋没材を提供することを目的とする。
【００２０】
【課題を解決するための手段】
本発明は、耐熱材と半水石膏を含む歯科用高温鋳造用石膏系埋没材において、石膏の減水剤として温度６００℃で燃え抜ける減水剤を含有することを特徴とする歯科用高温鋳造用石膏系埋没材を提供するものである。
【００２１】
【発明の実施の形態】
上記耐熱材としては、特に限定はないが、好ましくはＭｇＯーＡｌ_２Ｏ_３スピネルやアルミナ等が用いられる。このうちＭｇＯーＡｌ_２Ｏ_３スピネル（マグネシア／アルミナ系スピネル）は、例えば７００℃、８００℃、或いは９００℃以上というような高温における鋳造に際しても、鋳造製品に鋳造不良が生じない石膏系鋳造埋没材であり（特願平８ー２６９３５９号）、本発明における耐熱材としても特に好ましく用いられる。
【００２２】
上記マグネシア／アルミナ系スピネルにより、例えば温度１３５０〜１４００℃、或いはそれ以上というような高温鋳造に際しても焼き付けや鋳巣が生じず、目的とする鋳造体について鋳造不良のない（脆化や汚染されることのない）石膏系埋没材とすることができる。この耐熱材は歯科用に使用されるプレシャス（貴金属）系、セミプレシャス系、或いはＮｉーＣｒ系などの高融点金属を鋳造するに際して高温鋳造特性に優れ、鋳込み不良を防止し、鋳物の表面の円滑性等の特性を確保することができる。またＭｇＯーＡｌ_２Ｏ_３スピネルは、それ自体石膏埋没材用耐熱材として優れた特性を有し、金属化合物等を添加することなしに石膏の分解を抑制することができ、このため添加成分の量を減らすことができる。
【００２３】
ＭｇＯーＡｌ_２Ｏ_３スピネルの粒度については、面粗度（面粗度を損なわない）の観点から粒径が２０μｍ（ミクロンメートル）以下であるのがよく、また平均気孔率増加（鋳造性の安定性）の観点から、その平均粒子径が８μｍ以上であるのがよい。その粒径が２０μｍ程度を超えると鋳造面に荒れが生じ、例えば粒径３０μｍのものを使用すると、そのスピネルフィラーの粗粒性により鋳造面の表面性状（バリ発生、表面粗さ）に欠陥が生じる。なお粒径２０μｍ以下とは、面粗度を損なわない範囲でそれより大きい粒径のものが幾分含まれていても差し支えなく、本明細書中粒径２０μｍ以下とはその許容範囲を含めた意味である。
【００２４】
本発明は、上記のような耐熱材とともに、結合材として半水石膏を使用する。そしてこれらを含む歯科用石膏系鋳造埋没材において、石膏の減水剤として温度６００℃で燃え抜ける減水剤を含有することを特徴とするものである。なお、本発明の歯科用石膏系鋳造埋没材において、耐熱材、結合材としての石膏及び減水剤のほか、適宜の補助成分を含有する場合を含むことはもちろんである。
【００２５】
減水剤は石膏系鋳造埋没材において混水量を少なくするために加えられるが、本発明における減水剤は、このような減水剤それ自体としての役目を果たすとともに、温度６００℃を下回る温度で燃えて該鋳造埋没材から抜出る性質を有する物質である。その例としては好ましくは有機高分子物質が挙げられるが、その燃焼特性を有するものであれば、合成によるものとは限らず天然のものであってもよい。
【００２６】
表２にその幾つかの例を示すが、これらとは限らず、減水剤それ自体としての役目を果たし、温度６００℃を下回る温度で燃えて該鋳造埋没材から抜出る物質であれば使用される。本発明では、これによって高温鋳造性を改善し、耐高温分解性に優れた歯科用高温鋳造用石膏系埋没材が現実に得られる。その効果の理由としては、埋没材に高溶合金の鋳造溶湯を流し込むときに、減水剤が温度６００℃を下回る温度で燃えて該鋳造埋没材から抜出ることにより多数の細孔を生じ、これが前記式（１）〜（３）に示す反応により発生するガスの抜け道となること及びその燃え抜けにより埋没体（鋳造埋没材）中にカーボンが残らないため、石膏の分解温度を下げる還元雰囲気となることが避けられることによるものと推認されるが、さらに詳細な理由は現時点では不明である。
【００２７】
【表 ２】

【００２８】
【実施例】
以下、実施例に基づき本発明をさらに詳しく説明するが、本発明がこれら実施例により制限されないことはもちろんである。本実施例においては、耐熱材としてアルミナ（実施例１）又はＭｇＯーＡｌ_２Ｏ_３スピネル（実施例２〜７、比較例１〜４）を用い、結合材としてα石膏を用いた組成物を使用して鋳造金属を作製し、それらについての試験結果を記載している。
【００２９】
《実施例１〜７、比較例１〜４》
表３に示すとおり、耐熱材として所定粒径のアルミナ及び各種粒径のスピネル（ＭｇＯーＡｌ_２Ｏ_３スピネル）とα石膏を成分とする組成物を調合した。なお、減水剤の添加量は耐熱材とα石膏との合計量１００重量部に対するものである。次いで、これらの各調合物を用いて図１中（ｂ）〜（ｅ）に示すような工程で高融点金属を鋳造した場合について、鋳造性、表面形状を試験した。表３はこれら試験の結果である。
【００３０】
表３中「鋳造性」及び「表面形状」の欄中におけるＮｉーＣｒ系とは、鋳造金属として市販の陶材焼付用ＮｉーＣｒ系の合金（ＮｉーＣｒ系の合金の溶融温度は通常１３１５〜１３４３℃程度であり、この範囲を下回るものや上回るものもあるが、本例ではＮｉ：８０ｗｔ％、Ｃｒ：１５ｗｔ％、残余Ｍｏ等で、溶融温度＝１３３０℃のもの）を用いた場合であり、ＰＲ系とは鋳造金属として市販のＡｕーＰｄ系の貴金属合金（例えば陶材焼付用としては各種のものがあり、通常Ａｕが８０ｗｔ％以上で、溶融温度は１１７０〜１２７５℃程度の範囲であり、この範囲を下回るものや上回るものもあるが、本例ではＡｕ：８２ｗｔ％、残余Ｐｄ等の組成で、溶融温度＝１２４５℃のもの）を用いた場合である。なお、表３の粒子径（μｍ）の欄中「−２０」、「−３０」とはそれぞれ篩下の意味である（−はアンダーの意味）。
【００３１】
表３における各項目の評価基準は以下のとおりである。まず「鋳造性」については、再現性の良否及び石膏の熱分解に起因する鋳巣（巣、穴）発生の有無を観察したもので、再現性がよく、巣、穴等の鋳造欠陥が全くない場合を◎印、◎印のものに比べれば再現性にやや欠ける場合を○印、巣、穴等の鋳造不良が多い場合を△印、巣、穴等の鋳造不良が生じる場合を×印とした。
【００３２】
次に、製品の表面形状中の「バリ発生」については、再現性が良く、バリの発生がない場合を◎印、バリの発生がある場合を△印、バリが数多く発生する場合を×印とし、製品の表面形状中「表面粗さ」については、面荒れがなく、表面が滑らかな場合を◎印、表面の滑らかさが◎印のもの程ではないが、これに準じるものを○印、部分的に面荒れがある場合を△印、面荒れが全面にある場合を×印とした。
【００３３】
まず、実施例１〜８は減水剤として燃え抜け温度が６００℃以下のものを用いた場合であるが、何れも鋳造性に優れた製品が得られている。これに対して、比較例１は減水剤として燃え抜け温度が８００℃の▲５▼アルキルナフタレンスルホン酸ホルマリン縮合物を用いた場合、比較例２は減水剤として燃え抜け温度が８００℃の▲６▼縮合ナフタレンスルホン酸のナトリウム塩を用いた場合、比較例３では減水剤として燃え抜け温度が７００℃の▲７▼βーナフタレンスルホン酸のホルマリン縮合物ナトリウム塩を用いた場合であるが、これらの場合には、ＮｉーＣｒ系で鋳造性×印、すなわち巣、穴等の鋳造不良が生じてしまい、ＰＲ系でも鋳造性△印、すなわち多くの巣、穴等の鋳造不良が見られた。
【００３４】
次に、製品の表面形状については、ＮｉーＣｒ系及びＰＲ系ともに、実施例１〜７の何れの場合も「バリ発生」については再現性が良くバリの発生がなく（◎印）、「表面粗さ」については面荒れがなく表面が滑らかであった（◎印）。これらの点は比較例１〜３でも同様であるが、上記のとおりこれら比較例１〜３では鋳造性の点で劣り充分ではない。また、実施例８は耐熱材であるスピネルの粒子径を−３０μｍ以下としたが、燃え抜け温度５４０℃の減水剤を用いた例である。この場合には、バリは認められるが、数多くはなく（△印）、「表面粗さ」についても部分的に面荒れはあるが（△印）、鋳造性に関しては再現性がよく、巣、穴等の鋳造欠陥は全くなかった（◎印）。このように、実施例８では鋳造性について格段に改善されており、これは減水剤として燃え抜け温度が６００℃以下のものを用いたことによる効果である。
【００３５】
【表 ３】

【００３６】
また、表４は実施例２、実施例６及び実施例７における試験終了後における埋没材について気孔径の分布、平均気孔率及び気孔率を測定したものである。表４から明らかなとおり、例えば気孔率については、実施例２では５３．５５％、実施例６では５４．３３％、実施例７では５１．３３％というように高い気効率が得られており、また気孔径分布についてもミクロン単位の気孔径が程よく分布していることが分かる。これは温度６００℃以下で燃え抜ける減水剤の作用によるものであるが、本発明によれば、このようにして高温鋳造性に優れ、バリ発生がなく、表面が滑らかな製品が再現性よく得られる。
【００３７】
【表 ４】

【００３８】
【発明の効果】
以上のとおり、本発明によれば、歯科用高温鋳造用石膏系埋没材において、耐熱材と結合材としての半水石膏を含み、該石膏の減水剤として温度６００℃以下で燃え抜ける減水剤を使用することにより、この石膏系埋没材を用いた鋳造時の高温鋳造性を格段に改善でき、バリ発生や面荒れがない優れた鋳造製品が得られる。
【図面の簡単な説明】
【図１】人工歯冠を作製する場合を例としたロストワックス精密鋳造法の概略を示す図。
【符号の説明】
１ 型（例：歯型）
２ 鋳造用パターン
３ 鋳型用材料
４ 除去工程前のワックス型
５ ワックスが除去されて形成された空洞部分[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a gypsum-based investment material for dental high-temperature casting, and more specifically, comprises a heat-resistant material as a main component and gypsum hemihydrate as a binder, and has a specific combustion characteristic (temperature characteristic) as a water reducing agent for the gypsum. The present invention relates to a gypsum-based investment material for high-temperature dental casting, which has excellent high-temperature decomposition resistance and has improved high-temperature castability, which comprises a water reducing agent having the formula (1).
[0002]
[Prior art]
Conventionally, there are various casting methods for synthetic resin, metal, and the like, and one of them is a lost wax method. This method has also been applied to the production of dental metal prostheses (such as crowns, bridges, inlays, etc.) that require precision. FIG. 1 is a view schematically showing a lost wax precision casting method for producing an artificial crown as an example [Gakuhodo Shuppan Co., Ltd., “Ceramic Engineering Handbook”, 1989, p. 1586]. In FIG. 1 (a), reference numeral 1 denotes a tooth mold, and the tooth mold 1 is made of an impression material from a model obtained from an impression master sample collected in the oral cavity, for example. As the material of the impression material, a gypsum-based, metal-based, resin-based, cement-based, or the like is used.
[0003]
The wax is heat-softened and injected onto the surface of the tooth mold 1 made of the above-mentioned material to form a casting pattern 2 made of wax, and the casting pattern 2 is transferred to a burying step. FIG. 1B shows the burying step (burying step). The casting pattern 2 is suspended in a container as shown in the figure, and a casting material (= investment material) 3 is poured into the casting pattern 2 as a slurry, preferably as a slurry. The casting material, that is, the investment material 3 is composed of a heat-resistant material such as quartz or cristobalite and a binder such as gypsum, phosphate, or silica gel.
[0004]
After the investment material 3 is solidified, the process proceeds to a step of melting or burning the wax constituting the casting pattern 2 to remove the wax. FIG. 1C shows a state before the wax removing step, and FIG. 1D shows a state after the wax removing step. 1 (c) to 1 (d), a reference numeral 4 denotes a mold formed from the consolidated investment material 3, and a reference numeral 5 in FIG. 1 (d) denotes a hollow portion formed by removing wax (that is, a hollow mold). ). Next, a melt of a Ni—Cr-based, precious, or semi-precious metal is cast into the cavity mold 5.
[0005]
FIG. 1E shows a state in which the melt is cast. After the completion of the casting step, the casting is cooled, the investment material (mold material) 3 is removed (that is, the cast molded body is dug out), and the cast molded body is taken out. FIG. 1 (f) shows a state before the processing and polishing step, and FIG. 1 (g) shows a product obtained after the step, that is, an artificial crown in the case of this explanation example. The above is the so-called indirect method, in which the investment material is directly injected into the impression matrix collected in the oral cavity to make an investment material model, a wax model is built on it, and the wax model is removed. There is also known a direct method in which an investment material is further poured into the wax model and the wax model is buried in the mold to form a mold (Japanese Patent Application Laid-Open No. 50-113417).
[0006]
As the mold material in the lost wax method as described above, that is, the investment material 3 (= the mold after the wax removing step) is roughly classified into a gypsum-based investment material using gypsum as a binder and a phosphate as a binder. There is a phosphate-based investment material that uses, and each has advantages and disadvantages as shown in Table 1.
[0007]
[Table 1]

[0008]
Among these, a gypsum-based investment material using a gypsum-based binder has excellent operability (fluidity) and excavation properties, and is excellent in various points such as no deformation or change with time due to residual stress. However, inferior heat resistance, especially when the casting temperature is high, gas is generated by the thermal decomposition of gypsum, which causes baking and cavities, and the target casting is embrittled or contaminated. There is a problem that it has a negative side characteristic that cannot be obtained. It is known that gypsum decomposes at a temperature of about 800 ° C. or 850 ° C., for example, depending on its type, and it is known that gypsum decomposition is accelerated particularly by the reduction of the gypsum composite due to the presence of a water reducing agent. Have been.
[0009]
Therefore, as an investment material applicable to the lost wax method, for example, in the case of an alloy having a relatively low melting point such as a gold alloy or a silver alloy (that is, a low melting alloy), a gypsum investment material is used. In the case of precious metal alloys for baking materials (precious system = PR system), semi-precious system alloys for baking ceramics, or alloys with relatively high melting points such as Ni-Cr alloys (that is, high melting alloys), A phosphate-based investment material using a phosphate-based binder is used.
[0010]
As described above, the reason why the phosphate investment material is used as a mold material for a high-melting alloy is that high-temperature casting characteristics, that is, no gas is generated during high-temperature casting, and that high-melting alloy is used as shown in Table 1. This is because a large casting shrinkage can be compensated for by a large expansion during the setting of the phosphate investment material. However, this phosphate-based investment material is difficult to operate because it uses a highly viscous colloidal silica solution at the time of use, and since the coagulation and expansion are not uniform, the wax pattern may be deformed due to residual stress, If the strength of the mold is too large, there are disadvantages such as damage to the target cast during excavation.
[0011]
On the other hand, in the case of a conventional gypsum-based investment material, a mold made from this investment material has appropriate strength and can be dug out without damaging the cast body. When contacted, the gypsum is thermally decomposed to generate gas, which has the disadvantage that it has physical properties that cannot be fundamentally exceeded, such as causing baking and cavities. For this reason, various techniques have been devised to overcome this problem, such as Japanese Patent Application Laid-Open Nos. 50-113417 and 61-20547, and Japanese Patent Publication No. 53-19877.
[0012]
However, the casting temperature in each of these techniques is at most 750 ° C. or less (the only one touched to a temperature of 750 ° C. is the case of JP-A-50-113417 only), and cristobalite, quartz, Alternatively, since SiO ₂ components such as silica stone are used, when this is applied for casting of a high melting alloy such as a precious alloy, for example, a reaction between the cast metal and the SiO ₂ component and a decomposition reaction of the gypsum component There is a problem that the surface of a cast product is roughened due to generation of gas due to the above. In addition, in this case, when the SiO ₂ component having a particle size of 50 μm or more is used, not only the surface condition of the casting is remarkably deteriorated due to the coarse component, but also the size of the casting becomes small, and the compatibility is poor. Problem.
[0013]
Further, in the investment material composition disclosed in JP-A-63-141906, alumina or the like is used as a refractory material, and natural starch and carbides and nitrides of transition metals of Groups IV, V and VI of the periodic table are used as an expanding agent. By adding one or more of boride, silicide, and sulfide, expansion is slow and shrinkage upon cooling is reduced, and it is described that compensation for casting shrinkage of the alloy is further improved. However, even in this case, when hemihydrate gypsum is used as the binder, gas remains due to the thermal decomposition of the gypsum during casting, which still causes problems such as poor casting of the casting.
[0014]
Thus, in any case, at the time of casting of a high melting point metal applied at a high temperature, in addition to the good operability of gypsum itself, castability, surface roughness required for gypsum-based investment material, No technique is disclosed that also satisfies the excavation property, the dimensional compatibility of the cast metal, and the like. In this regard, the present inventors have found that adding a cobalt compound, a titanium compound, a tungsten compound, a nickel compound, a chromium compound, a manganese compound, a zinc compound, a selenium compound, or a tin compound to a gypsum-based investment material, A gypsum-based investment material composition capable of suppressing the decomposition of gypsum generated during casting and preventing casting defects has been developed (Japanese Patent Application No. 8-29123), thereby satisfying the suitability for dimensional change in the casting. Can be.
[0015]
However, in order to satisfy this, it is necessary to increase the amount of these components, but if the amount is increased, the surface of the cast body may be roughened. Therefore, the present inventors have further focused on magnesia / alumina-based spinel as a heat-resistant material component of the gypsum cast investment material, and by using this as a heat-resistant material, suppresses the decomposition of gypsum and improves the surface state. Good operability and excavation properties, while maintaining the original properties of gypsum-based investment materials such as no deformation or change over time, eliminating the problem of high-temperature casting properties, that is, thermal expansion, and improving high-temperature decomposition resistance, etc. (Japanese Patent Application No. 8-269359).
[0016]
By the way, it is known that at the time of casting, the reaction of the following formulas (1) and (2) occurs mainly due to the influence of carbon such as residual wax, and the decomposition method of the formula (3) is adopted. For this reason, especially when the casting is performed at a temperature of 1400 ° C. or more and the casting is kept at a high temperature for a long time in the investment material, it is impossible to suppress the thermal decomposition of the gypsum. Caused poor casting.
[Formula 1]
CaSO ₄ + 4C → CaS + 4CO (1)
3CaSO ₄ + CaS → 4CaO + 4SO ₂ ↑ (2)
_{4CaSO 4 + 4C → 4CaO + 4CO} ↑ + 4SO 2 ↑ (3)
[0017]
As another method of eliminating the casting defect, a method of improving gas permeability (porosity) in the investment material and improving gas outflow is considered. For this reason, Japanese Patent Publication No. 52-11921 and Japanese Patent Application Laid-Open No. 54-104694 have taken into account the particle size of the filler used. Among these, in the gypsum investment for dental metal floor casting disclosed in Japanese Patent Publication No. 52-11921, a powder composed of 50-60% of silica stone powder, 40-50% of ultra-hard hemihydrate gypsum powder, and 1-2% of soluble chloride powder. The mixture is kneaded with water to form a slurry. The silica powder is pulverized to a particle size of 100 to 200 mesh 10% or less, and a particle size of 200 to 325 mesh 15% or more and passed through a 325 mesh sieve and 75% or less. Is used.
[0018]
Further, in Japanese Patent Application Laid-Open No. 54-104694, although the ternary silica powder, the ultra-hard hemihydrate gypsum powder and the soluble chloride powder have somewhat different ranges of the quantitative ratio, a particle size distribution of 200 to 325 mesh 25 to A silica powder having a weight of 60 to 75% by weight passing through a 40% by weight, 325 mesh sieve is used, and the ultra-hard hemihydrate gypsum powder has the same particle size distribution as the silica powder. However, these methods have not yet improved high-temperature castability.
[0019]
[Problems to be solved by the invention]
The present invention is a gypsum-based investment material for dental high-temperature casting containing hemihydrate gypsum as a heat-resistant material and a binder, by using a water-reducing agent that burns out at 600 ° C. or less as a water-reducing agent for the gypsum added thereto. An object of the present invention is to provide a gypsum-based investment material for high-temperature dental casting, which has improved high-temperature castability and excellent high-temperature decomposition resistance.
[0020]
[Means for Solving the Problems]
The present invention provides a gypsum-based investment material for dental high-temperature casting containing a heat-resistant material and hemihydrate gypsum, characterized in that it contains a water-reducing agent that burns off at a temperature of 600 ° C. as a water-reducing agent for gypsum. It provides a system investment.
[0021]
BEST MODE FOR CARRYING OUT THE INVENTION
The heat-resistant material is not particularly limited, but is preferably MgO-Al ₂ O ₃ spinel, alumina, or the like. Among them, MgO-Al ₂ O ₃ spinel (magnesia / alumina-based spinel) is a gypsum cast burial that does not cause casting defects in cast products even when casting at a high temperature such as 700 ° C., 800 ° C., or 900 ° C. or more. (Japanese Patent Application No. 8-269359) and is particularly preferably used as a heat-resistant material in the present invention.
[0022]
Due to the magnesia / alumina-based spinel, there is no baking or porosity even during high-temperature casting at a temperature of, for example, 1350 to 1400 ° C. or higher, and there is no casting defect in the target casting (brittleness and contamination). Gypsum investment). This heat-resistant material has excellent high-temperature casting properties when casting refractory metals such as precious (precious metal), semi-precious, and Ni-Cr alloys used for dentistry, prevents poor casting, and improves the surface of the casting. Characteristics such as smoothness can be secured. MgO-Al ₂ O ₃ spinel itself has excellent properties as a heat-resistant material for gypsum investment material, and can suppress the decomposition of gypsum without adding a metal compound or the like. The amount can be reduced.
[0023]
Regarding the particle size of the MgO-Al ₂ O ₃ spinel, the particle size is preferably 20 μm (micrometer) or less from the viewpoint of surface roughness (the surface roughness is not impaired), and the average porosity increase (castability). From the viewpoint of (stability), the average particle size is preferably 8 μm or more. If the particle size exceeds about 20 μm, the casting surface becomes rough. For example, when a particle having a particle size of 30 μm is used, defects are caused in the surface properties (burr generation, surface roughness) of the casting surface due to the coarseness of the spinel filler. Occurs. The particle size of 20 μm or less may include some particles having a larger particle size as long as the surface roughness is not impaired. In this specification, the particle size of 20 μm or less includes the allowable range. Meaning.
[0024]
The present invention uses hemihydrate gypsum as a binder together with the above-mentioned heat-resistant material. And the dental gypsum cast investment material containing these contains a water reducing agent which burns off at a temperature of 600 ° C. as a water reducing agent for gypsum. In addition, it goes without saying that the dental gypsum cast investment investment material of the present invention includes a case where it contains appropriate auxiliary components in addition to the heat-resistant material, the gypsum and the water reducing agent as the binder.
[0025]
The water reducing agent is added in order to reduce the amount of water mixed in the gypsum cast investment material, but the water reducing agent in the present invention functions as such a water reducing agent itself, and burns at a temperature below 600 ° C. It is a substance having the property of being extracted from the casting investment material. An example thereof is preferably an organic polymer substance. However, as long as it has the combustion characteristics, it is not limited to a synthetic substance and may be a natural substance.
[0026]
Some examples are shown in Table 2, but are not limited thereto, and any substance can be used as long as it functions as a water reducing agent itself and burns at a temperature lower than 600 ° C and is extracted from the casting investment material. You. According to the present invention, a high-temperature castability is improved, and a gypsum-based investment material for high-temperature dental casting excellent in high-temperature decomposition resistance is actually obtained. The reason for this effect is that, when pouring a high-melting alloy casting metal into the investment material, the water reducing agent burns at a temperature lower than 600 ° C. and exits from the investment material to produce a large number of pores. Since there is no escape path for the gas generated by the reaction shown in the above formulas (1) to (3) and no carbon remains in the buried body (cast investment material) due to its burning, a reducing atmosphere for lowering the decomposition temperature of gypsum It is presumed that this would be avoided, but the detailed reason is unknown at this time.
[0027]
[Table 2]

[0028]
【Example】
Hereinafter, the present invention will be described in more detail with reference to Examples, but it goes without saying that the present invention is not limited by these Examples. In this example, a composition using alumina (Example 1) or MgO-Al ₂ O ₃ spinel (Examples 2 to 7 and Comparative Examples 1 to 4) as a heat-resistant material and α gypsum as a binder was used. It was used to make cast metals and the test results for them are described.
[0029]
<< Examples 1 to 7, Comparative Examples 1 to 4 >>
As shown in Table 3, as a heat-resistant material, a composition containing alumina having a predetermined particle size, spinel having various particle sizes (MgO-Al ₂ O ₃ spinel), and α gypsum was prepared. The amount of the water reducing agent is based on 100 parts by weight of the total amount of the heat-resistant material and the α-gypsum. Next, the castability and the surface shape were tested when a high melting point metal was cast by the steps shown in (b) to (e) in FIG. 1 using each of these formulations. Table 3 shows the results of these tests.
[0030]
In Table 3, "Ni-Cr" in the column of "Castability" and "Surface Shape" means a Ni-Cr alloy for porcelain baking commercially available as a casting metal (the melting temperature of the Ni-Cr alloy is usually The temperature is about 1315 to 1343 ° C., and some are below or above this range, but in this example, Ni: 80 wt%, Cr: 15 wt%, residual Mo, etc., and the melting temperature = 1330 ° C.) The PR type is a commercially available Au-Pd type noble metal alloy as a casting metal (for example, there are various types for baking porcelain, and usually, Au is 80 wt% or more, and the melting temperature is about 1170 to 1275 ° C. In this example, the composition is Au: 82 wt%, the balance is Pd, etc., and the melting temperature is 1245 ° C.). In the column of the particle diameter (μm) in Table 3, “−20” and “−30” respectively mean “under sieve” (− means under).
[0031]
The evaluation criteria for each item in Table 3 are as follows. First, “castability” was obtained by observing the quality of reproducibility and the occurrence of voids (cavities and holes) due to the thermal decomposition of gypsum. When there is no casting, ◎ mark, when the reproducibility is slightly lower than that of ◎ mark, ○ mark, when there are many casting defects such as nests, holes, etc. And
[0032]
Next, regarding the “burr generation” in the surface shape of the product, the reproducibility is good and the mark ◎ indicates that no burr occurred, Δ indicates that burr occurred, and x indicates that many burr occurred. Regarding the “surface roughness” in the surface shape of the product, the mark ◎ indicates that the surface is smooth and the surface is smooth, and the smoothness of the surface is not as good as the mark ◎. △ indicates that the surface was partially rough, and X indicates that the surface was rough.
[0033]
First, Examples 1 to 8 are cases in which a burn-out temperature of 600 ° C. or less was used as a water reducing agent, and all of them obtained products excellent in castability. On the other hand, in Comparative Example 1, when a burn-out temperature of 800 ° C. was used as a water reducing agent, (5) an alkylnaphthalenesulfonic acid formalin condensate was used, and in Comparative Example 2, a burn-out temperature of 800 ° C. was used as a water reducing agent. ▼ When the sodium salt of the condensed naphthalene sulfonic acid is used, in Comparative Example 3, the burn-out temperature is 700 ° C. as the water reducing agent. ７ The sodium salt of the formalin condensate of β-naphthalene sulfonic acid is used. In the case of (1), castability × marks, ie, poor casting such as nests, holes, etc., occurred in the Ni—Cr system, and castability Δ marks, ie, poor casting, such as many cavities, holes, etc., were also observed in the PR system. .
[0034]
Next, regarding the surface shape of the product, in both of the Ni-Cr-based and PR-based products, in any of Examples 1 to 7, "burr generation" was excellent in reproducibility and no burr was generated (marked with ◎). As for “surface roughness”, the surface was smooth without surface roughness (marked with ◎). These points are the same in Comparative Examples 1 to 3, but as described above, Comparative Examples 1 to 3 are inferior in castability and are not sufficient. Example 8 is an example in which the particle diameter of spinel, which is a heat-resistant material, was -30 μm or less, but a water reducing agent having a burn-out temperature of 540 ° C. was used. In this case, burrs are observed, but there are not many (marked with 、), and although “surface roughness” is partially rough (marked with △), reproducibility is good with regard to castability, There were no casting defects such as holes (marked with ◎). As described above, in Example 8, the castability was remarkably improved, and this is an effect due to the use of a water reducing agent having a burnout temperature of 600 ° C. or less.
[0035]
[Table 3]

[0036]
Table 4 shows the distribution of the pore diameter, the average porosity, and the porosity of the investment material after the test in Example 2, Example 6, and Example 7 were completed. As is evident from Table 4, for example, the porosity is as high as 53.55% in Example 2, 54.33% in Example 6, and 51.33% in Example 7. It can also be seen that the pore size distribution in microns is moderately distributed. This is due to the action of the water reducing agent that burns out at a temperature of 600 ° C. or lower. According to the present invention, a product with excellent high-temperature castability, no burr generation, and a smooth surface can be obtained with good reproducibility. Can be
[0037]
[Table 4]

[0038]
【The invention's effect】
As described above, according to the present invention, a gypsum-based investment material for dental high-temperature casting contains a heat-resistant material and a hemihydrate gypsum as a binder, and a water reducing agent that burns out at a temperature of 600 ° C. or lower as a water reducing agent for the gypsum. By using this, the high-temperature castability at the time of casting using this gypsum-based investment material can be remarkably improved, and an excellent cast product without burrs or surface roughness can be obtained.
[Brief description of the drawings]
FIG. 1 is a view schematically showing a lost wax precision casting method in which an artificial crown is produced as an example.
[Explanation of symbols]
Type 1 (example: tooth type)
2 Casting pattern 3 Molding material 4 Wax mold before removal process 5 Cavity formed by removing wax

Claims (3)

A gypsum-based investment material for high-temperature dental casting, comprising a gypsum-based investment material for dental high-temperature casting containing a heat-resistant material and hemihydrate gypsum, which contains a water-reducing agent that burns off at a temperature of 600 ° C. as a gypsum water-reducing agent. The refractory material is MgO over Al ₂ O ₃ spinel powder in a claim 1 dental hot casting gypsum-based investing material according. The MgO over Al ₂ O ₃ and an average particle diameter in the particle size 20μm or less of the spinel powder is more powders 8μm claim 2 dental hot casting gypsum-based investing material according.

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