JP2004298746A - Method for reusing excessively sprayed water paint - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for reusing melamine resin-curable type water paint, at the concentrating step of which a melamine resin or a pigment does not agglomerate in the water paint and by which the curability of the water paint to be reused and such film characteristics of a cured coating film as hardness, water resistance and weather resistance are hardly deteriorated. <P>SOLUTION: This water paint to be reused contains a binder component containing the melamine resin as a curing agent. Booth circulating water contains 1-10 wt.% water-compatible organic solvent and the pH of booth the circulating water is adjusted to 8.0-9.0. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【００４０】
によりポリエステル樹脂のブース循環水に対する流出率（％）が算出される。使用するポリエステル樹脂の種類は、水性であれば特に限定されないが、流出率が１０％以下であることが好ましい。
【００４１】
次いで、上記ポリエステル樹脂とメラミン樹脂とを水性塗料と同一の量比で含有する混合樹脂ワニスを用意する。混合樹脂ワニスに、水又は純水に水混和性有機溶媒を加えた水溶液を加えて固形分濃度１０重量％に希釈する。限外ろ過膜を用いて、加えた量と同じ量のろ液を得られるまで、希釈混合樹脂ワニスを濃縮する。得られたろ液の固形分を測定する。式
【００４２】
【数５】

【００４３】
によりメラミン樹脂のブース循環水に対する流出率（％）が算出される。
【００４４】
メラミン樹脂の流出率を使えば過剰塗料を捕集及び濃縮する過程で失われるメラミン樹脂量を算出でき、濃縮塗料に残存するメラミン樹脂量を決定できる。そうすると、再使用混合塗料に含まれるメラミン樹脂の量は、初期塗料の補充量が決まれば決定される。初期塗料の補充量は塗装装置の塗着効率に依存して定まる。それゆえ、使用する塗装装置の塗着効率を測定すれば、その塗着効率から初期塗料の補充量が決定される。このようにして、再使用混合塗料に含まれるメラミン樹脂の量はメラミン樹脂の流出率と塗着効率から決定することができる。
【００４５】
再使用混合塗料の硬化性低下を防止するためには、このように決定された再使用混合塗料に含まれるメラミン樹脂の量を、硬化に必要な所定量以上に保つように塗料組成を設計すればよい。この量は、メラミン樹脂の含有量が初期塗料の７０％以上になる量であることが解っている。
【００４６】
このように、初期塗料の補充量は塗装装置の被塗物に対する塗着効率に依存して定まる。また、正常な硬度の塗膜を形成するために必要な量のメラミン樹脂の量も経験的に解っている。そのため、正常な硬度の塗膜を形成するために必要な量のメラミン樹脂を再使用混合塗料中に確保するために、メラミン樹脂が有すべき流出率の最大値は、塗装装置の塗着効率基準にして計算することができる。
【００４７】
ポリエステル樹脂の流出率も考慮した場合の一般的計算式
▲１▼初期塗料中のポリエステル樹脂の重量をＡとし、ポリエステル樹脂とメラミン樹脂の固形分重量比を、式Ａ／（１００−Ａ）とすると、硬化性の低下のない再使用可能な混合塗料の（ポリエステル樹脂＋メラミン樹脂）１００中に含まれるメラミン樹脂含有量は、式０．７×（１００−Ａ）以上でなければならない。ここで、０．７を乗じているのは、硬化に必要なメラミン樹脂の量が初期塗料の７０％以上であることが解っているからである。
【００４８】
▲２▼また、塗着効率がＸ％の塗装装置（塗装ライン）であれば再使用混合塗料中の初期塗料と回収塗料の混合比は、式Ｘ／（１００−Ｘ）となる。
【００４９】
▲３▼ここで、ポリエステル樹脂の流出率をＰ％として、メラミン樹脂の流出率がＹ％である初期塗料を濃縮して得られる回収塗料中のメラミン樹脂含有量は、式
（１００−Ａ）×（１００−Ｙ）／１００
となり、回収塗料中のポリエステル樹脂含有量は、式
Ａ×（１００−Ｐ）／１００
となる。
【００５０】
▲４▼したがって、再使用する混合塗料中の（ポリエステル樹脂＋メラミン樹脂）１００中に含まれるメラミン樹脂含有量Ｃは、式
【００５１】
【数６】

【００５２】
となり、ポリエステル樹脂含有量Ｂは、式
【００５３】
【数７】

【００５４】
となる。
【００５５】
▲１▼および▲２▼より以下の関係式が成り立つ
【００５６】
【数８】

【００５７】
ここで、１００−Ａ＝α、１００−Ｘ＝βとおくと、上記関係式よりメラミン流出率Ｙは、式
【００５８】
【数９】

【００５９】
となる。このようにして、所定の塗着効率とポリエステル樹脂の流出率がわかれば、ポリエステル樹脂とメラミン樹脂がいかなる固形分量比であってもメラミン樹脂の最大流出率を理論的に求めることができる。
【００６０】
ポリエステル樹脂の流出率を無視した場合の簡易的計算式
ポリエステ樹脂の流出率は１０％以下であり実質上無視して計算してもよい。その場合の簡易的計算式は以下の通りである。
【００６１】
▲１▼初期塗料中のポリエステル樹脂とメラミン樹脂の固形分重量比を、式Ａ／（１００−Ａ）とすると、硬化性の低下のない再使用可能な混合塗料の（ポリエステル樹脂＋メラミン樹脂）１００中に含まれるメラミン樹脂含有量は、式０．７×（１００−Ａ）以上でなければならない。
【００６２】
▲２▼また、塗着効率がＸ％の塗装装置（塗装ライン）であれば再使用混合塗料中の初期塗料と回収塗料の混合比は、式Ｘ／（１００−Ｘ）となる。
【００６３】
▲３▼ここで、ポリエステル樹脂の流出率＝０％として、メラミン樹脂の流出率がＹ％である初期塗料を濃縮して得られる回収塗料中のメラミン樹脂含有量は、式
（１００−Ａ）×（１００−Ｙ）／１００
となる。
【００６４】
▲４▼したがって、再使用する混合塗料中の（ポリエステル樹脂＋メラミン樹脂）１００中に含まれるメラミン樹脂含有量は、式
【００６５】
【数１０】

【００６６】
となる。
▲１▼および▲２▼より以下の関係式が成り立つ
【００６７】
【数１１】

【００６８】
この式を整理計算すると、メラミン樹脂の最大流出率Ｙは、式
【数１２】
Ｙ＝３０００／（１００−Ｘ） （１）
［式中、Ｙは最大流出率（％）であり、Ｘは塗着効率（％）である。］
となる。
【００６９】
但し、上述の理論式以外にも、ポリエステル樹脂の流出率の値にかかわらず利用できる実験式がある。すなわち、メラミン樹脂の最大流出率は、式
【数１３】
Ｙ＝０．０００１Ｘ^３−０．００６５Ｘ^２＋０．５６３５Ｘ＋４０ （２）
［式中、Ｙは最大流出率（％）であり、Ｘは塗着効率（％）である。］
によって計算してもよい。
【００７０】
一例を示せば、以下の表に示す関係が成立する。
【００７１】
【表１】

【００７２】
このようにして、塗着効率に適した流出率のメラミン樹脂を選択して水性塗料の硬化剤として使用すれば再使用塗料の硬化性低下は生じない。
【００７３】
更に、メラミン樹脂の流出率はメラミン樹脂の溶解性パラメータ（ＳＰ値）と相関性を有しており、溶解性パラメータに換算することができる。この関係を利用すれば、流出率よりもより一般的な特性値である溶解性パラメータを基準にしてメラミン樹脂を選択することができる。つまり、式（１）又は式（２）で計算される最大流出率の値を溶解性パラメータに換算し、その値以下の溶解性パラメータを有するメラミン樹脂を選択して使用すればよい。
【００７４】
流出率と溶解性パラメータとの関係は、式
【数１４】
Ｚ＝０．０６０４Ｙ＋１０．５
［式中、Ｚは溶解性パラメータであり、Ｙは流出率（％）である。］
によって表される。
【００７５】
【実施例】
次に、本発明を実施例によって具体的に説明するが、本発明はこれらに限定されない。実施例において「部」、「％」及び「比率」は特に断らないかぎり、重量基準である。
【００７６】
製造例
ポリエステル樹脂の合成
【表２】

【００７７】
撹拌機、温度調節機、冷却管を備えた反応容器に表１に示す成分を仕込み、昇温した。反応により生成する水をキシレンと共沸させて除去した。
【００７８】
還流開始より約２時間をかけて温度を１９０℃にし、カルボン酸相当の酸価が４０、及び水酸基価が９９になるまで撹拌と脱水を継続し、反応を終了した。さらにジメチルエタノールアミン６．０部を加え、脱イオン水１１３部を加えた。得られたポリエステル樹脂は数平均分子量３０００であり、水溶液の固形分濃度は４４％であった。
【００７９】
実施例１
水性塗料の調製
製造例で得たポリエステル樹脂ワニス３７．７部、脱イオン水１０部、二酸化チタン（石原産業社製「ＣＲ−９７（商品名）」）８．０部、及び沈降性硫酸バリウム（堺化学社製「Ｂ−３４（商品名）」）３１．０部、カーボンブラック（三菱化学社製「ＭＡ−１００（商品名）」）１．０部をペイントコンディショナーに仕込み、粒径１．４〜２．０ｍｍのガラスビーズ９０部を加えて、室温で１時間ミル分散した。分散物を金網に通してガラスビーズを除去して顔料ペーストを得た。顔料ペーストに含まれている顔料の粒度は１０μｍ以下であった。
【００８０】
得られた顔料ペースト８７．７部、製造例で得たポリエステル樹脂ワニス５７．８部、メラミン樹脂（三井サイテック社製「マイコート７２３（商品名）」、固形分１００％）１８部、表面調整剤（エアプロダクツジャパン社製「サーフィノール１０４Ｅ（商品名）」）０．５部、脱イオン水１０部をディスパーに仕込み、１０分間撹拌して水性塗料を得た（塗料配合＝１７４ｇ）。
【００８１】
ポリエステル樹脂とメラミン樹脂との固形分比は７／３であり、塗料の固形分濃度は５８％であった。また、つぶゲージ法（ＪＩＳ Ｋ ５４００Ａ法）により顔料の粒度を測定したところ１０μｍ以下であった。
【００８２】
得られた水性塗料を希釈及び膜濃縮する試験を行った。まず、水性塗料５００ｇにブチルジグリコールの５％水溶液１５００ｇを加えて固形分濃度１５％になるまで希釈した。その後、ここにジメチルエタノールアミンを加えてｐＨ８．５に調節した。次いで、希釈塗料を液送ポンプ付きペンシル型ＵＦモジュール（旭化成社製）に循環させて希釈液と同量（１５００ｇ）のろ液が生成するまで濃縮した。顔料の粒度を測定したところ１０μｍ以下であった。
【００８３】
比較例１
水性塗料５００ｇに脱イオン水１５００ｇを加えて固形分濃度１５％になるまで希釈した。次いで、希釈塗料を液送ポンプ付きペンシル型ＵＦモジュール（旭化成社製）に循環させて希釈液と同量（１５００ｇ）のろ液が生成するまで濃縮した。顔料の粒度を測定したところ２５μｍ以上であった。
【００８４】
比較例２
ブチルジグリコールの１５％水溶液にジメチルエタノールアミンを加えてｐＨ８．５に調節した。水性塗料５００ｇにこの水溶液１５００ｇを加えて固形分濃度１５％になるまで希釈した。次いで、希釈塗料を液送ポンプ付きペンシル型ＵＦモジュール（旭化成社製）に循環させて希釈液と同量（１５００ｇ）のろ液が生成するまで濃縮した。この濃縮には６時間を要し、実施例１で行った場合の３倍の時間が必要であり、濃縮効率が悪化した。
【００８５】
実施例２
メラミン樹脂の種類を三井サイテック社製「サイメル３７０（商品名）」（固形分濃度８８％）に変更し、これを２０．５部配合すること以外は実施例１と同様にして水性塗料を得た。ポリエステル樹脂とメラミン樹脂との固形分比は７／３であり、塗料の固形分濃度は５７％であった。また、顔料の粒度は１０μｍ以下であった。次いで、実施例１と同様にして塗料の希釈濃縮試験を行った。結果を表３に示す。
【００８６】
比較例３
実施例２で得られた水性塗料を用いること以外は比較例１と同様にして塗料の希釈濃縮試験をそれぞれ行った。結果を表３に示す。
【００８７】
実施例３
メラミン樹脂の種類を三井サイテック社製「マイコート２１２（商品名）」（固形分濃度９０％）に変更し、これを２０部配合すること以外は実施例１と同様にして水性塗料を得た。ポリエステル樹脂とメラミン樹脂との固形分比は７／３であり、塗料の固形分濃度は５７％であった。また、顔料の粒度は１０μｍ以下であった。次いで、実施例１と同様にして塗料の希釈濃縮試験を行った。結果を表３に示す。
【００８８】
比較例４
実施例３で得られた水性塗料を用いること以外は比較例１と同様にして塗料の希釈濃縮試験をそれぞれ行った。結果を表３に示す。
【００８９】
実施例４
メラミン樹脂の種類を三井サイテック社製「サイメル２３８（商品名）」（固形分濃度１００％）に変更し、これを１８部配合すること以外は実施例１と同様にして水性塗料を得た。ポリエステル樹脂とメラミン樹脂との固形分比は７／３であり、塗料の固形分濃度は５８％であった。また、顔料の粒度は１０μｍ以下であった。次いで、実施例１と同様にして塗料の希釈濃縮試験を行った。結果を表３に示す。
【００９０】
比較例５
実施例４で得られた水性塗料を用いること以外は比較例１と同様にして塗料の希釈濃縮試験をそれぞれ行った。結果を表３に示す。
【００９１】
【表３】

【００９２】
表３に示されるように、５重量％の量でブチルジグリコール（ＢＤＧ）を含み、ｐＨ８．５に調節されているブース循環水で希釈及び濃縮すると、顔料の粒径は増加しなかった。
【００９３】
実施例５
流出率の測定
製造例で得たポリエステル樹脂ワニス（固形分濃度４４％）５００ｇにブチルジグリコールの５％水溶液１７００ｇを加えて固形分濃度１０％になるまで希釈した。ここにジメチルエタノールアミンを加えてｐＨ８．０に調節した。この溶液を、液送ポンプ付きペンシル型ＵＦモジュール（旭化成社製）に循環させて濃縮し、ろ液１７００ｇを得た。ろ液の固形分濃度は０．８６％であった。この値からポリエステル樹脂流出率を計算すると６．６％であった。
【００９４】
メラミン樹脂として、三井サイテック社製の「マイコート５０８（商品名）」（固形分濃度８０％）を用いた。製造例で得たポリエステル樹脂ワニスとこのメラミン樹脂とを、ポリエステル樹脂とメラミン樹脂との固形分比が７／３になるように混合し、固形分濃度４０％になるように純水で希釈した。
【００９５】
得られた混合ワニス５００ｇにブチルジグリコールの５％水溶液１５００ｇを加えて固形分濃度１０％になるまで希釈した。ここにジメチルエタノールアミンを加えてｐＨ８．０に調節した。この溶液を、液送ポンプ付きペンシル型ＵＦモジュール（旭化成社製）に循環させて濃縮し、ろ液１５００ｇを得た。ろ液の固形分濃度は０．６７％であった。この値及びポリエステル樹脂流出率からメラミン樹脂流出率を計算すると１．３％であった。
【００９６】
上記と同様にして、異なる種類のメラミン樹脂の流出率を測定した。結果を表４に示す。
【００９７】
【表４】

ａ，ｂ）三井サイテック社製メラミン樹脂の商品名
【００９８】
参考例１
リン酸亜鉛処理した縦１５０ｍｍ、横７０ｍｍ、厚さ０．７ｍｍの溶融亜鉛めっき鋼板に、カチオン電着塗料（日本ペイント社製「パワートップＵ−５０（商品名）」）を乾燥膜厚２０μｍになるように電着塗装し、１６０℃で３０分間焼付けて硬化させた。
【００９９】
次いで、電着塗膜の上に、実施例１で得られた希釈濃縮試験前の水性塗料をエアスプレーにて乾燥膜厚２５μｍになるように塗布し、１５０℃で３０分間焼き付け硬化させた。得られた塗膜の硬度は鉛筆硬度Ｈであった。
【０１００】
実施例６
実施例１で得られた水性塗料はメラミン樹脂として三井サイテック社製「マイコート７２３（商品名）」を使用している。この樹脂の流出率は６６％である。塗着効率が６０％の塗装装置を想定すると、使用できるメラミン樹脂の流出率は、表１で求めたように算出すると７２％以下である。
【０１０１】
実施例１で得られた希釈濃縮試験後の水性塗料（回収塗料）４０ｇに、実施例１で得られた希釈濃縮試験前の水性塗料（初期塗料）６０ｇを加えて再使用混合塗料を得た。この再使用混合塗料中希釈濃縮試験前の水性塗料（初期塗料）の含有量は６０％である。尚、この含有量は塗装装置の塗着効率に相当する。次いで、水性塗料としてこの再使用混合塗料を用いること以外は参考例１と同様にして塗膜を形成した。得られた塗膜の硬度は鉛筆硬度でＨであった。
【０１０２】
比較例６
実施例１で得られた希釈濃縮試験後の水性塗料（回収塗料）７５ｇに、実施例１で得られた希釈濃縮試験前の水性塗料（初期塗料）２５ｇを加えて再使用混合塗料を得た。この再使用混合塗料中希釈濃縮試験前の水性塗料（初期塗料）の含有量は２５％である。次いで、水性塗料としてこの再使用混合塗料を用いること以外は参考例１と同様にして塗膜を形成した。すなわち、実施例１で用いたメラミンと同一のメラミンを用いて、塗着効率が２５％である場合を比較例とした。得られた塗膜の硬度は鉛筆硬度でＦであった。
【０１０３】
参考例２
リン酸亜鉛処理した縦１５０ｍｍ、横７０ｍｍ、厚さ０．７ｍｍの溶融亜鉛めっき鋼板に、カチオン電着塗料（日本ペイント社製「パワートップＵ−５０（商品名）」）を乾燥膜厚２０μｍになるように電着塗装し、１６０℃で３０分間焼付けて硬化させた。
【０１０４】
次いで、電着塗膜の上に、実施例２で得られた希釈濃縮試験前の水性塗料（初期塗料）をエアスプレーにて乾燥膜厚２５μｍになるように塗布し、１５０℃で３０分間焼き付け硬化させた。得られた塗膜の硬度は鉛筆硬度Ｈであった。
【０１０５】
実施例７
実施例２で得られた水性塗料はメラミン樹脂として三井サイテック社製「サイメル３７０（商品名）」を使用している。この樹脂の流出率は５７％である。塗着効率が４０％の塗装装置を想定すると、使用できるメラミン樹脂の流出率は５９％以下である。
【０１０６】
実施例２で得られた希釈濃縮試験後の水性塗料（回収塗料）４０ｇに、実施例２で得られた希釈濃縮試験前の水性塗料（初期塗料）６０ｇを加えて再使用混合塗料を得た。この再使用混合塗料中希釈濃縮試験前の水性塗料（初期塗料）の含有量は６０％である。次いで、水性塗料としてこの再使用混合塗料を用いること以外は参考例２と同様にして塗膜を形成した。得られた塗膜の硬度は鉛筆硬度でＨであった。
【０１０７】
比較例７
実施例２で得られた希釈濃縮試験後の水性塗料（回収塗料）７５ｇに、実施例２で得られた希釈濃縮試験前の水性塗料（初期塗料）２５ｇを加えて再使用混合塗料を得た。この再使用混合塗料中希釈濃縮試験前の水性塗料（初期塗料）の含有量は２５％である。次いで、水性塗料としてこの再使用混合塗料を用いること以外は参考例２と同様にして塗膜を形成した。すなわち、実施例２で用いたメラミンと同一のメラミンを用いて、塗着効率が２５％である場合を比較例とした。得られた塗膜の硬度は鉛筆硬度でＦであった。
【０１０８】
参考例３
リン酸亜鉛処理した縦１５０ｍｍ、横７０ｍｍ、厚さ０．７ｍｍの溶融亜鉛めっき鋼板に、カチオン電着塗料（日本ペイント社製「パワートップＵ−５０（商品名）」）を乾燥膜厚２０μｍになるように電着塗装し、１６０℃で３０分間焼付けて硬化させた。
【０１０９】
次いで、電着塗膜の上に、実施例３で得られた希釈濃縮試験前の水性塗料（初期塗料）をエアスプレーにて乾燥膜厚２５μｍになるように塗布し、１５０℃で３０分間焼き付け硬化させた。得られた塗膜の硬度は鉛筆硬度Ｈであった。
【０１１０】
【発明の効果】
メラミン樹脂硬化系水性塗料の再使用方法によれば、濃縮する過程において水性塗料中で樹脂や顔料が凝集せず、再使用する塗料については硬化性が低下したり、硬化塗膜の硬度、耐水性、耐候性等の膜特性が低下することはない。
【図面の簡単な説明】
【図１】本発明の方法に利用できる単色水性塗料用のリサイクルシステムの構成を模式的に表す図面である。
【符号の説明】
１…塗装ブース、２…塗装ガン、４…過剰噴霧塗料、５…ブース循環水槽、６…ブース循環水、７…濃縮槽、８…濃縮槽の内容物、９…限外濾過装置、１０…濾液槽、１１…濾液、１２…回収塗料槽、１３…濃縮塗料、２０…水性中塗り塗料、３０…洗浄槽、３１…洗浄水、５０…被塗物、７０…気流[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for recovering and reusing an aqueous paint that has not been applied to an object to be coated in a spray coating method, and particularly when spray coating an automobile body or the like, the aqueous paint that has not been applied to the vehicle body. It relates to a method of collecting and reusing.
[0002]
[Prior art]
When spraying a paint on an object such as an automobile body in a paint booth, a lot of paint dust not applied to the object is generated, and this paint dust is dissolved or dispersed in washing water in the paint booth. Collected. Since a large amount of paint dust is collected in the washing water as described above, if it is discarded as it is, the paint is lost, and also the problem of environmental pollution is caused.
[0003]
Therefore, it has been studied to collect and reuse the paint collected in the washing water. For example, a recovery method disclosed in JP-A-49-51324 (Patent Document 1) is provided. Have been. That is, the water-based paint composition is obtained by collecting the sprayed water-based paint composition into water and filtering the paint dilution water obtained through a reverse osmosis filtration membrane or an ultrafiltration membrane. The concentration of non-volatile components such as water-soluble resins and pigments, such as water-soluble alkyd resins and water-soluble acrylic resins for coating film formation, is reduced to the same level as that of the original aqueous solution. It can be recycled so as to approximate a coating composition.
[0004]
However, in a melamine resin-cured water-based paint, the melamine resin is water-soluble, and when water is filtered for recovery, the melamine resin is also filtered together with the water, and the content of the melamine resin in the recovered water-based paint is reduced. Become smaller. As a result, the curability of the paint to be reused is reduced, the hardness of the cured coating film, water resistance, and film properties such as weather resistance are deteriorated. Resin and pigment tend to aggregate.
[0005]
JP-A-5-214292 (Patent Document 2) discloses a method for solving this problem by controlling the SP value of a resin as a binder component. However, in this method, the types of water-based paints that can be reused are limited to those containing a specific binder resin, and are not necessarily effective for melamine resin-cured water-based paints.
[0006]
Until now, in melamine resin-cured water-based paints, what kind of melamine resin is easy to flow out, and how much the outflow amount will adversely affect the film properties of the cured coating film has been clearly studied. There was no operation that relied on empirical rules when reusing. In the future, in order to develop a wide range of reuse methods for water-based paints, the relationship between the amount of melamine resin eluted and the change in the properties of the coating film will be clarified, and an index that will allow selection of the appropriate melamine resin according to the application and the characteristics of the coating line Need to be established.
[0007]
[Patent Document 1]
JP-A-49-51324
[Patent Document 2]
JP-A-5-214292
[0008]
[Problems to be solved by the invention]
The present invention is to solve the above conventional problems, the purpose is, in the process of concentration, the resin or pigment does not agglomerate in the aqueous coating, the curability of the reused paint is reduced, It is an object of the present invention to provide a method for reusing a melamine resin-cured water-based coating material without deteriorating film properties such as hardness, water resistance, and weather resistance of a cured coating film.
[0009]
[Means for Solving the Problems]
The present invention provides a step of spraying the water-based paint on the object to be coated;
Collecting the oversprayed waterborne paint dust with booth circulating water;
Separating the collected aqueous paint and the paint diluent containing the booth circulating water into a concentrate and a filtrate using an ultrafiltration membrane;
Using the filtrate as the booth circulating water and using the concentrate as the water-based paint.
The water-based paint contains a binder component having a melamine resin as a curing agent, the booth circulating water contains a water-miscible organic solvent in an amount of 1 to 10% by weight, and is adjusted to pH 8.0 to 9.0; The object of the present invention is to provide a method for reusing a water-based paint, whereby the above object is achieved.
[0010]
BEST MODE FOR CARRYING OUT THE INVENTION
In the method of the present invention, a melamine resin-cured aqueous paint is used. A melamine resin-cured water-based paint is a paint containing a binder component obtained by combining a base resin and a melamine resin, and a pigment and an additive generally used in a water-based paint for automotive body coating in a state of being dissolved or dispersed in an aqueous medium. Say. Preferred as the base resin is an aqueous polyester resin.
[0011]
The polyester resin is produced by condensing a polyhydric carboxylic acid component and a polyhydric alcohol component. However, a small proportion of monocarboxylic acid, hydroxycarboxylic acid, lactone, alkyd resin and the like may be simultaneously condensed.
[0012]
The polycarboxylic acid component includes aromatic polycarboxylic acids such as phthalic anhydride, isophthalic acid, terephthalic acid, trimellitic anhydride, tetrabromophthalic anhydride, tetrachlorophthalic anhydride, and pyromellitic anhydride; hexahydrophthalic anhydride Acid, tetrahydrophthalic anhydride, 1,4- and 1,3-cyclohexanedicarboxylic acid, maleic anhydride, succinic anhydride, adipic acid, sebacic acid, azelaic acid; 4-hydroxybenzoic acid, hydroxypivalic acid, 12-hydroxy Hydroxycarboxylic acids such as stearic acid; and monocarboxylic acids such as benzoic acid and t-butylbenzoic acid can be used.
[0013]
As the polyhydric alcohol component, diols and trivalent or higher polyols can be used. Preferred are hydroxyl-terminated polyalkadiene diols derived from conjugated alkadiene oligomers or polymers having a degree of polymerization of 5 to 50 (molecular weight of 1,000 to 4,000, particularly 1,500 to 3,000), , 4-polyisoprene diol, 1,4- and 1,2-polybutadiene diol, and hydrogenated products thereof.
[0014]
The polyester resin may be an alkyd resin containing a drying or semi-drying oil and their fatty acids. The drying oil or semi-drying oil and the fatty acids thereof may be fatty acids having 6 or more carbon atoms or fats and oils containing the same, up to 30% by weight, preferably up to 25% by weight, especially 10 to 20% by weight of the polyester resin. It is contained in the constituting amount. Examples of these fat components are castor oil, linseed oil, dehydrated castor oil, tung oil, safflower oil, soybean oil, tall oil, coconut oil, palm kernel oil and their fatty acids. Palm oil and palm kernel oil are preferred.
[0015]
Polyester resins further include monoepoxide compounds such as Cardura E (trade name: manufactured by Ciel Chemical) and lactones (β-propiolactone, dimethylpropiolactone, butyrolactone, γ-valerolactone, ε-caprolactone, γ- Caprolactone, γ-caprylolactone, crotlactone, δ-valerolactone, δ-caprolactone, etc.). In particular, lactones form a polyester chain by ring-opening addition of a polyhydric carboxylic acid and a polyhydric alcohol to the polyester chain, and further serve to improve the chipping resistance of the aqueous intermediate coating composition. These may be present in 3 to 30%, preferably 5 to 20%, especially 7 to 15%, of the total weight of all reaction components.
[0016]
The polyester resin can be synthesized, for example, by heating and condensing a polycarboxylic acid component, a polyhydric alcohol component and the above-mentioned reaction components in a nitrogen stream at, for example, 150 to 250 ° C. for 4 to 10 hours. In that case, you may use the catalyst normally used for a condensation reaction.
[0017]
The conditions of the condensation reaction are such that the resulting polyester resin has an acid value of 5 to 150 mg KOH / g (solid content), preferably 10 to 100 mg KOH / g (solid content), and a hydroxyl value of 30 to 250 mg KOH / g (solid content), Should be adjusted to reach 50 to 150 mg KOH / g (solids) and a weight average molecular weight of 10,000 to 200,000, preferably 25,000 to 160,000.
[0018]
The water-based paint may further contain, in addition to the polyester resin, a fine powder of a resin containing a carboxyl group, for example, an acrylic resin or a polyester resin containing a carboxyl group. The fine powder of such a resin has a particle size of 3 to 60 μm, particularly 5 to 25 μm, and an acid value of 15 to 300 mgKOH / g (solid content), particularly 30 to 100 mgKOH / g (solid content). It can be used preferably. By dispersing such resin fine particles, not only the water-based paint can be made high solid, but also the sagging limit film thickness can be improved by the structural viscosity effect.
[0019]
The amount of the resin fine powder to be added is such that the weight ratio of the resin fine powder to the polyester resin is 2/98 to 40/60, preferably 5/95 to 30/70, particularly 10/90 to 25/75. It is.
[0020]
The melamine resin is a cross-linking agent, and when heated, reacts with a hydroxyl group or the like contained in a base resin such as a polyester resin and cures it. It is preferable that the melamine resin also reacts with a fine powder of a resin optionally added. Specific examples of the melamine resin include di-, tri-, tetra-, penta-, hexa-methylol melamine and their alkyl ethers (alkyl is methyl, ethyl, propyl, isopropyl, butyl, isobutyl, etc.). Can be. The preferred number average molecular weight Mn of the melamine resin is 300 to 3000, and if the number average molecular weight Mn is less than 300, the curability of the coating film may be insufficient, and if it exceeds 3000, the aqueous solution may be insufficient. There is. More preferably, it is 300-2000.
[0021]
Commercial products of melamine resin include “Mycoat (trade name) 506”, “Mycoat 212”, “Mycoat 723”, “Cymel (trade name) 238”, “Symel 202”, and “Symel 202” manufactured by Mitsui Cytec. 254 "," CYMEL 211 "," CYMER 303 "," CYMER 370 "," CYMER 325 "," CYMER 1156 ", etc.," Mitsubishi Chemical Co., Ltd. "" Yuban (trade name) 20N "," Yuvan 20SB "," Yuvan 128 " And hydrophilic melamine and / or hydrophobic melamine resins such as “Sumimar (trade name) M-50W”, “Sumimar M-40N”, and “Sumimar M-30W” manufactured by Sumitomo Chemical Co., Ltd.
[0022]
The content of the melamine resin in the aqueous paint is such that the solid content weight ratio of the polyester resin and the melamine resin is 50/50 to 95/5, preferably 60/40 to 90/10. If the weight ratio is less than 50/50, the chipping resistance of the coating film decreases. When the weight ratio exceeds 95/5, the curability of the coating material is reduced, and the film properties of the cured coating film are deteriorated.
[0023]
The aqueous medium for dissolving or dispersing the polyester resin is usually deionized water.If necessary, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, propylene glycol monomethyl ether, propylene glycol monoethyl ether, ethylene glycol monomethyl ether, ethylene glycol monobutyl ether , A small amount of a water-miscible organic solvent such as methanol, ethanol, isopropyl alcohol, n-butyl alcohol, sec-butyl alcohol, t-butyl alcohol, dimethylformamide, and N-methylpyrrolidone.
[0024]
The bases used include inorganic bases such as alkali metal hydroxides and aqueous ammonia, and methylamine, dimethylamine, trimethylamine, ethylamine, diethylamine, triethylamine, isopropylamine, diisopropylamine, diethylenetriamine, triethylenetetramine, monoethyleneamine. There are amines such as ethanolamine, diethanolamine, 2-amino-2-methylpropanol, morpholine, N-methylmorpholine, N-ethylmorpholine, piperazine, dimethylethanolamine, diethylethanolamine, and dimethyldodecylamine. Triethylamine, dimethylethanolamine and diethylethanolamine are preferred.
[0025]
Aqueous paints preferably used in the present invention include conventional pigments such as titanium dioxide, lead white, zinc white, zinc sulfide, graphite, carbon black, rust preventive pigments, and optionally calcium carbonate, barium sulfate, and silicate. System, silicate system, aluminum hydrate system and the like. The aqueous paint most preferably used in the present invention is a mixture of titanium dioxide as a white pigment and carbon black as a black pigment.
[0026]
The pigment content of the coating composition is expressed as a weight ratio of pigment / solid resin in the coating composition, and is usually 0.01 / 1 to 1/1, preferably 0.03 / 1 to 0.9 /. Within the range of 1.
[0027]
The method of the present invention can be performed, for example, in a recycling system for water-based paint as shown in FIG. FIG. 1 shows a recycling system for a single-color water-based paint. First, the water-based intermediate coating 20 is sprayed from the coating gun 2 in the coating booth 1 to spray-coat an object 50 (an automobile body is assumed). At this time, the excess spray paint 4 that has not adhered to the article 50 is collected by the booth circulating water 6 and sent to the booth circulating water tank 5 by the airflow 70 from the ceiling. Booth circulating water 6 is used repeatedly.
[0028]
After a certain coating period, all or part (preferably all) of the contents in the booth circulating water tank 5 is transferred to the concentration tank 7. The contents 8 of the concentration tank 7 are usually separated by an ultrafiltration device 9 into a filtrate 11 and a concentrated paint. The separated filtrate 11 is transferred to and stored in a filtrate tank 10, while the concentrated paint is returned to the concentration tank 7 again, further separated and concentrated by the ultrafiltration device 9, and finally has a desired concentration. The concentrated paint 13 which has reached the point is transferred to the recovery paint tank 12 and stored.
[0029]
The concentrated paint 13 is taken out when necessary, replenished with a new water-based paint to reduce coating loss, sent to the coating gun 2 and reused for painting. On the other hand, the filtrate 11 is transferred to the booth circulating water tank 5 and reused for washing in the booth 1 and for the booth circulating water 6, or transferred to the washing tank 30 to be washed water 31 after concentration and separation. Used for cleaning the outer filtration device 9.
[0030]
In the method of the present invention, the booth circulating water 6 preferably contains a water-miscible organic solvent in an amount of 1 to 10% by weight, particularly 2 to 8% by weight. This is to improve the stability of the paint (paint diluent) in the booth circulating water. As the water-miscible organic solvent, butyl diglycol, ethylene glycol monobutyl ether and the like are particularly preferable. If the content of the water-miscible organic solvent is less than 1% by weight, the paint will agglomerate or the stability of the paint diluent will decrease after concentration, and if it exceeds 10% by weight, the stability of the concentrated paint will decrease. The filtration efficiency at the time of external filtration concentration decreases.
[0031]
The booth circulating water 6 is preferably adjusted to pH 8.0 to 9.0, particularly preferably 8.2 to 8.8. This is for ensuring the dilution stability of the amine-neutralized paint. If the pH of the booth circulating water is less than 8.0, the stability of the paint diluent decreases, or the paint diluent agglomerates during concentration, and if it exceeds 9.0, the hydrolysis resistance of the resin decreases significantly, The stability of the paint is reduced and the quality of the coating film is reduced.
[0032]
The pH of the booth circulating water is adjusted to contain a base. As the base, those described above can be used. Among them, particularly preferred bases are dimethylethanolamine and triethylamine.
[0033]
Since the concentrated paint 13 is film-concentrated, if a part of the melamine resin is eluted in the booth circulating water, the curability is reduced. Therefore, if this is reused for coating as it is, there is a high possibility that a cured coating film having poor film properties such as hardness, water resistance and weather resistance will be formed. However, in practice, a reused mixed paint obtained by replenishing a concentrated paint with a new water-based paint is reused for painting. Therefore, the depletion of the melamine resin eluted from the concentrated paint can be compensated for by the melamine resin contained in the water-based paint (initial paint) to a certain extent.
[0034]
If the sum of the amount of melamine resin contained in the initial paint and the amount of melamine resin remaining in the concentrated paint is an amount sufficient to cure the polyester resin of the re-use mixed paint, re-use The curability of the paint as a whole does not decrease, and the deterioration of film properties is also prevented.
[0035]
The amount of the melamine resin contained in the initial paint to be mixed is calculated by multiplying the replenishment amount of the initial paint by the content of the melamine resin. The replenishment amount of the initial paint is almost the same as the coating decrement, and the amount is determined when the coating efficiency of the coating device is determined.
[0036]
The amount of melamine resin remaining in the concentrated paint is an amount obtained by subtracting the amount of melamine resin eluted from the amount of melamine resin contained in the initial paint.
[0037]
The amount of the eluted melamine resin can be calculated by multiplying the amount of the melamine resin contained in the water-based paint, if the amount of the melamine resin and the outflow rate of the melamine resin to the booth circulating water is determined. Here, the melamine resin may be hydrophilic or hydrophobic, and it is not possible to determine from the characteristics of the melamine resin alone how much it flows out under the specific conditions of collecting and concentrating excess paint. Have difficulty. However, the outflow rate of the melamine resin to the booth circulating water can be determined by, for example, devising the following method.
[0038]
First, a polyester resin varnish having a concentration equal to or lower than that of the aqueous paint (for example, a nonvolatile content of 40%) is prepared. An aqueous solution obtained by adding a water-miscible organic solvent to water or pure water is added to a polyester resin varnish to dilute to a solid concentration of 10% by weight. The diluted polyester resin varnish is concentrated using an ultrafiltration membrane until the same amount of filtrate is obtained. The solid content concentration of the obtained filtrate is measured. formula
[0039]
(Equation 4)

[0040]
Thus, the outflow rate (%) of the polyester resin to the booth circulating water is calculated. The type of the polyester resin used is not particularly limited as long as it is aqueous, but the outflow rate is preferably 10% or less.
[0041]
Next, a mixed resin varnish containing the polyester resin and the melamine resin in the same quantitative ratio as the aqueous paint is prepared. An aqueous solution obtained by adding water or a water-miscible organic solvent to water or pure water is added to the mixed resin varnish to dilute to a solid concentration of 10% by weight. Using a ultrafiltration membrane, concentrate the diluted mixed resin varnish until the same amount of filtrate is obtained. The solid content of the obtained filtrate is measured. formula
[0042]
(Equation 5)

[0043]
Thus, the outflow rate (%) of the melamine resin with respect to the booth circulating water is calculated.
[0044]
By using the outflow rate of the melamine resin, the amount of melamine resin lost in the process of collecting and concentrating excess paint can be calculated, and the amount of melamine resin remaining in the concentrated paint can be determined. Then, the amount of the melamine resin contained in the reused mixed paint is determined if the replenishment amount of the initial paint is determined. The replenishment amount of the initial paint is determined depending on the coating efficiency of the coating apparatus. Therefore, if the coating efficiency of the coating apparatus used is measured, the replenishment amount of the initial paint is determined from the coating efficiency. In this way, the amount of the melamine resin contained in the reused mixed paint can be determined from the outflow rate of the melamine resin and the coating efficiency.
[0045]
In order to prevent a decrease in the curability of the reused mixed paint, the paint composition should be designed so that the amount of the melamine resin contained in the reused mixed paint thus determined is maintained at a predetermined amount or more necessary for curing. Just fine. It has been found that this amount is such that the content of the melamine resin becomes 70% or more of the initial paint.
[0046]
As described above, the replenishment amount of the initial paint is determined depending on the efficiency of application of the coating apparatus to the workpiece. Further, the amount of the melamine resin required to form a coating film having a normal hardness is empirically known. Therefore, in order to secure the necessary amount of melamine resin in the reused mixed paint to form a coating film having a normal hardness, the maximum value of the effusion rate that the melamine resin should have is determined by the coating efficiency of the coating apparatus. It can be calculated based on the standard.
[0047]
General formula when considering the outflow rate of polyester resin
{Circle around (1)} Assuming that the weight of the polyester resin in the initial coating material is A and the weight ratio of the solid content of the polyester resin and the melamine resin is the formula A / (100-A), a reusable mixed coating material with no deterioration in curability. (Polyester resin + melamine resin) 100, the content of melamine resin must be at least 0.7 × (100−A). Here, the reason why the value is multiplied by 0.7 is that it is known that the amount of the melamine resin necessary for curing is 70% or more of the initial paint.
[0048]
{Circle around (2)} In the case of a coating apparatus (coating line) having a coating efficiency of X%, the mixing ratio between the initial paint and the recovered paint in the reused mixed paint is expressed by the formula X / (100−X).
[0049]
{Circle over (3)} Assuming that the outflow rate of the polyester resin is P%, the content of the melamine resin in the recovered paint obtained by concentrating the initial paint in which the outflow rate of the melamine resin is Y% is expressed by the following formula.
(100-A) × (100-Y) / 100
The polyester resin content in the recovered paint is calculated by the formula
A x (100-P) / 100
Becomes
[0050]
{Circle around (4)} Therefore, the melamine resin content C contained in the (polyester resin + melamine resin) 100 in the mixed paint to be reused is represented by the formula:
[0051]
(Equation 6)

[0052]
And the polyester resin content B is given by the formula
[0053]
(Equation 7)

[0054]
Becomes
[0055]
The following relational expressions hold from (1) and (2)
[0056]
(Equation 8)

[0057]
Here, assuming that 100−A = α and 100−X = β, the melamine outflow rate Y is calculated from the above relational expression by the following expression
[0058]
(Equation 9)

[0059]
Becomes In this manner, if the predetermined coating efficiency and the outflow rate of the polyester resin are known, the maximum outflow rate of the melamine resin can be theoretically obtained regardless of the solid content ratio of the polyester resin and the melamine resin.
[0060]
Simple calculation formula when the outflow rate of polyester resin is ignored
The outflow rate of the polyester resin is 10% or less and may be substantially ignored and calculated. The simple calculation formula in that case is as follows.
[0061]
{Circle around (1)} Assuming that the solid content weight ratio of the polyester resin and the melamine resin in the initial paint is expressed by the formula A / (100-A), (polyester resin + melamine resin) The melamine resin content contained in 100 must be equal to or more than the formula 0.7 × (100-A).
[0062]
{Circle around (2)} In the case of a coating apparatus (coating line) having a coating efficiency of X%, the mixing ratio between the initial paint and the recovered paint in the reused mixed paint is expressed by the formula X / (100−X).
[0063]
(3) Here, assuming that the outflow rate of the polyester resin is 0%, the melamine resin content in the recovered paint obtained by concentrating the initial paint in which the outflow rate of the melamine resin is Y% is expressed by the following formula.
(100-A) × (100-Y) / 100
Becomes
[0064]
{Circle over (4)} Therefore, the content of the melamine resin contained in the (polyester resin + melamine resin) 100 in the mixed paint to be reused is expressed by the formula
[0065]
(Equation 10)

[0066]
Becomes
The following relational expressions hold from (1) and (2)
[0067]
(Equation 11)

[0068]
When this equation is rearranged and calculated, the maximum outflow rate Y of the melamine resin is given by the equation
(Equation 12)
Y = 3000 / (100−X) (1)
[Wherein, Y is the maximum outflow rate (%) and X is the coating efficiency (%). ]
Becomes
[0069]
However, in addition to the theoretical formulas described above, there are empirical formulas that can be used regardless of the value of the outflow rate of the polyester resin. That is, the maximum outflow rate of melamine resin is given by the formula
(Equation 13)
Y = 0.0001X³-0.0065X²+ 0.5635X + 40 (2)
[Wherein, Y is the maximum outflow rate (%) and X is the coating efficiency (%). ]
May be calculated by
[0070]
For example, the relationship shown in the following table is established.
[0071]
[Table 1]

[0072]
In this manner, if a melamine resin having an outflow rate suitable for the coating efficiency is selected and used as a curing agent for a water-based paint, the curability of the reused paint does not decrease.
[0073]
Further, the outflow rate of the melamine resin has a correlation with the solubility parameter (SP value) of the melamine resin, and can be converted into the solubility parameter. By utilizing this relationship, the melamine resin can be selected based on the solubility parameter which is a more general characteristic value than the outflow rate. That is, the value of the maximum outflow rate calculated by the formula (1) or (2) is converted into a solubility parameter, and a melamine resin having a solubility parameter equal to or less than the value may be selected and used.
[0074]
The relationship between the outflow rate and the solubility parameter is given by the equation
[Equation 14]
Z = 0.0604Y + 10.5
Wherein Z is the solubility parameter and Y is the outflow rate (%). ]
Represented by
[0075]
【Example】
Next, the present invention will be described specifically with reference to examples, but the present invention is not limited thereto. In the examples, “parts”, “%” and “ratio” are based on weight unless otherwise specified.
[0076]
Manufacturing example
Synthesis of polyester resin
[Table 2]

[0077]
The components shown in Table 1 were charged into a reaction vessel equipped with a stirrer, a temperature controller, and a cooling pipe, and the temperature was raised. Water generated by the reaction was removed by azeotropic distillation with xylene.
[0078]
The temperature was brought to 190 ° C. over about 2 hours from the start of reflux, and stirring and dehydration were continued until the acid value equivalent to carboxylic acid became 40 and the hydroxyl value became 99, thereby terminating the reaction. Further, 6.0 parts of dimethylethanolamine were added, and 113 parts of deionized water were added. The obtained polyester resin had a number average molecular weight of 3000, and the solid content of the aqueous solution was 44%.
[0079]
Example 1
Preparation of water-based paint
37.7 parts of polyester resin varnish obtained in Production Example, 10 parts of deionized water, 8.0 parts of titanium dioxide ("CR-97 (trade name)" manufactured by Ishihara Sangyo Co., Ltd.), and precipitated barium sulfate (Sakai Chemical Co., Ltd.) 31.0 parts of "B-34 (trade name)" manufactured by Mitsubishi Chemical Corporation and 1.0 parts of "MA-100 (trade name)" manufactured by Mitsubishi Chemical Corporation were charged into a paint conditioner, and the particle size was 1.4 to 2 parts. 90 parts of glass beads having a diameter of 0.0 mm were added, and the mixture was mill-dispersed at room temperature for 1 hour. The dispersion was passed through a wire net to remove the glass beads to obtain a pigment paste. The particle size of the pigment contained in the pigment paste was 10 μm or less.
[0080]
87.7 parts of pigment paste obtained, 57.8 parts of polyester resin varnish obtained in Production Example, 18 parts of melamine resin (“Mycoat 723 (trade name)” manufactured by Mitsui Cytec, solid content 100%), surface conditioning 0.5 part of an agent ("Surfinol 104E (trade name)" manufactured by Air Products Japan) and 10 parts of deionized water were charged into a disper, and stirred for 10 minutes to obtain an aqueous coating material (coating compound = 174 g).
[0081]
The solid content ratio between the polyester resin and the melamine resin was 7/3, and the solid content concentration of the paint was 58%. When the particle size of the pigment was measured by a crush gauge method (JIS K 5400A method), it was 10 μm or less.
[0082]
A test for diluting and film-concentrating the obtained water-based paint was conducted. First, 1500 g of a 5% aqueous solution of butyl diglycol was added to 500 g of the water-based paint, and the mixture was diluted until the solid content concentration became 15%. Thereafter, dimethylethanolamine was added thereto to adjust the pH to 8.5. Next, the diluted paint was circulated through a pencil type UF module with a liquid feed pump (manufactured by Asahi Kasei Corporation) and concentrated until the same amount (1500 g) of filtrate as the diluent was produced. When the particle size of the pigment was measured, it was 10 μm or less.
[0083]
Comparative Example 1
To 500 g of the water-based paint, 1500 g of deionized water was added to dilute to a solid content concentration of 15%. Next, the diluted paint was circulated through a pencil type UF module with a liquid feed pump (manufactured by Asahi Kasei Corporation) and concentrated until the same amount (1500 g) of filtrate as the diluent was produced. When the particle size of the pigment was measured, it was 25 μm or more.
[0084]
Comparative Example 2
Dimethylethanolamine was added to a 15% aqueous solution of butyldiglycol to adjust the pH to 8.5. 1500 g of this aqueous solution was added to 500 g of the water-based paint, followed by dilution until the solid content concentration became 15%. Next, the diluted paint was circulated through a pencil type UF module with a liquid feed pump (manufactured by Asahi Kasei Corporation) and concentrated until the same amount (1500 g) of filtrate as the diluent was produced. This concentration required 6 hours, which was 3 times as long as that in Example 1, and the concentration efficiency was deteriorated.
[0085]
Example 2
A water-based paint was obtained in the same manner as in Example 1 except that the type of the melamine resin was changed to “Symel 370 (trade name)” (manufactured by Mitsui Cytec Co., Ltd.) (solid content: 88%), and 20.5 parts of this was blended. Was. The solid content ratio between the polyester resin and the melamine resin was 7/3, and the solid content concentration of the paint was 57%. The particle size of the pigment was 10 μm or less. Next, a dilution and concentration test of the paint was performed in the same manner as in Example 1. Table 3 shows the results.
[0086]
Comparative Example 3
A dilution and concentration test of the paint was performed in the same manner as in Comparative Example 1 except that the aqueous paint obtained in Example 2 was used. Table 3 shows the results.
[0087]
Example 3
A water-based paint was obtained in the same manner as in Example 1, except that the type of the melamine resin was changed to "Mycoat 212 (trade name)" (solid content concentration: 90%) manufactured by Mitsui Cytec Co., Ltd., and 20 parts of this was blended. . The solid content ratio between the polyester resin and the melamine resin was 7/3, and the solid content concentration of the paint was 57%. The particle size of the pigment was 10 μm or less. Next, a dilution and concentration test of the paint was performed in the same manner as in Example 1. Table 3 shows the results.
[0088]
Comparative Example 4
A dilution and concentration test of the paint was performed in the same manner as in Comparative Example 1 except that the aqueous paint obtained in Example 3 was used. Table 3 shows the results.
[0089]
Example 4
A water-based paint was obtained in the same manner as in Example 1 except that the type of the melamine resin was changed to “Cymel 238 (trade name)” (manufactured by Mitsui Cytec Co., Ltd.) (solid content concentration: 100%) and 18 parts of this was blended. The solid content ratio between the polyester resin and the melamine resin was 7/3, and the solid content concentration of the paint was 58%. The particle size of the pigment was 10 μm or less. Next, a dilution and concentration test of the paint was performed in the same manner as in Example 1. Table 3 shows the results.
[0090]
Comparative Example 5
A dilution and concentration test of the paint was performed in the same manner as in Comparative Example 1 except that the aqueous paint obtained in Example 4 was used. Table 3 shows the results.
[0091]
[Table 3]

[0092]
As shown in Table 3, dilution and concentration with booth circulating water containing butyl diglycol (BDG) in an amount of 5% by weight and adjusted to pH 8.5 did not increase the pigment particle size.
[0093]
Example 5
Measuring runoff rate
To 500 g of the polyester resin varnish (solid content: 44%) obtained in Production Example, 1700 g of a 5% aqueous solution of butyl diglycol was added to dilute the solid content to 10%. The pH was adjusted to 8.0 by adding dimethylethanolamine. This solution was circulated through a pencil type UF module with a liquid feed pump (manufactured by Asahi Kasei Corporation) and concentrated to obtain 1700 g of a filtrate. The solid content of the filtrate was 0.86%. When the polyester resin outflow rate was calculated from this value, it was 6.6%.
[0094]
As a melamine resin, "Mycoat 508 (trade name)" (solid content: 80%) manufactured by Mitsui Cytec Co., Ltd. was used. The polyester resin varnish obtained in the production example and this melamine resin were mixed so that the solid content ratio of the polyester resin and the melamine resin became 7/3, and diluted with pure water so that the solid content concentration became 40%. .
[0095]
To 500 g of the obtained mixed varnish, 1500 g of a 5% aqueous solution of butyl diglycol was added to dilute the mixture to a solid content of 10%. The pH was adjusted to 8.0 by adding dimethylethanolamine. This solution was circulated through a pencil-type UF module with a liquid feed pump (manufactured by Asahi Kasei Corporation) and concentrated to obtain 1500 g of a filtrate. The solid content of the filtrate was 0.67%. The melamine resin outflow rate calculated from this value and the polyester resin outflow rate was 1.3%.
[0096]
In the same manner as above, outflow rates of different types of melamine resins were measured. Table 4 shows the results.
[0097]
[Table 4]

a, b) Trade name of melamine resin manufactured by Mitsui Cytec
[0098]
Reference Example 1
A cationic electrodeposition paint ("Power Top U-50 (trade name)" manufactured by Nippon Paint Co., Ltd.) was applied to a hot-dip galvanized steel sheet having a length of 150 mm, a width of 70 mm and a thickness of 0.7 mm treated with zinc phosphate to a dry film thickness of 20 μm. It was then electrodeposited and cured by baking at 160 ° C. for 30 minutes.
[0099]
Next, the aqueous coating material before the dilution concentration test obtained in Example 1 was applied on the electrodeposition coating film by air spray so as to have a dry film thickness of 25 μm, and baked and cured at 150 ° C. for 30 minutes. The hardness of the obtained coating film was pencil hardness H.
[0100]
Example 6
The water-based paint obtained in Example 1 uses "Mycoat 723 (trade name)" manufactured by Mitsui Cytec as a melamine resin. The outflow rate of this resin is 66%. Assuming a coating apparatus having a coating efficiency of 60%, the outflow rate of melamine resin that can be used is 72% or less when calculated as calculated in Table 1.
[0101]
To 40 g of the water-based paint (recovered paint) after the dilution and concentration test obtained in Example 1 was added 60 g of the water-based paint (initial paint) before the dilution and concentration test obtained in Example 1 to obtain a reused mixed paint. . The content of the water-based paint (initial paint) before the dilution concentration test in the reused mixed paint is 60%. This content corresponds to the coating efficiency of the coating apparatus. Next, a coating film was formed in the same manner as in Reference Example 1 except that this reused mixed paint was used as an aqueous paint. The hardness of the obtained coating film was H in pencil hardness.
[0102]
Comparative Example 6
25 g of the water-based paint (initial paint) obtained in Example 1 before the dilution-concentration test was added to 75 g of the water-based paint (recovered paint) obtained in Example 1 after the dilution / concentration test to obtain a reused mixed paint. . The content of the water-based paint (initial paint) before the dilution / concentration test in the reused mixed paint was 25%. Next, a coating film was formed in the same manner as in Reference Example 1 except that this reused mixed paint was used as an aqueous paint. That is, a comparative example was made using the same melamine as that used in Example 1 and having a coating efficiency of 25%. The hardness of the obtained coating film was F in pencil hardness.
[0103]
Reference Example 2
A cationic electrodeposition paint ("Power Top U-50 (trade name)" manufactured by Nippon Paint Co., Ltd.) was applied to a hot-dip galvanized steel sheet having a length of 150 mm, a width of 70 mm and a thickness of 0.7 mm treated with zinc phosphate to a dry film thickness of 20 μm. It was then electrodeposited and cured by baking at 160 ° C. for 30 minutes.
[0104]
Next, the aqueous paint (initial paint) before the dilution concentration test obtained in Example 2 was applied on the electrodeposition coating film by air spray so as to have a dry film thickness of 25 μm, and baked at 150 ° C. for 30 minutes. Cured. The hardness of the obtained coating film was pencil hardness H.
[0105]
Example 7
The water-based paint obtained in Example 2 uses “Cymel 370 (trade name)” manufactured by Mitsui Cytec as a melamine resin. The outflow rate of this resin is 57%. Assuming a coating apparatus having a coating efficiency of 40%, the outflow rate of usable melamine resin is 59% or less.
[0106]
To 40 g of the water-based paint (recovery paint) after the dilution and concentration test obtained in Example 2 was added 60 g of the water-based paint (initial paint) before the dilution and concentration test obtained in Example 2 to obtain a reused mixed paint. . The content of the water-based paint (initial paint) before the dilution concentration test in the reused mixed paint is 60%. Next, a coating film was formed in the same manner as in Reference Example 2 except that this reused mixed paint was used as an aqueous paint. The hardness of the obtained coating film was H in pencil hardness.
[0107]
Comparative Example 7
To 75 g of the water-based paint (recovered paint) after the dilution and concentration test obtained in Example 2 was added 25 g of the water-based paint (initial paint) before the dilution and concentration test obtained in Example 2 to obtain a reused mixed paint. . The content of the water-based paint (initial paint) before the dilution / concentration test in the reused mixed paint was 25%. Next, a coating film was formed in the same manner as in Reference Example 2 except that this reused mixed paint was used as an aqueous paint. That is, a comparative example was made using the same melamine as that used in Example 2 and having a coating efficiency of 25%. The hardness of the obtained coating film was F in pencil hardness.
[0108]
Reference Example 3
A cationic electrodeposition paint ("Power Top U-50 (trade name)" manufactured by Nippon Paint Co., Ltd.) was applied to a hot-dip galvanized steel sheet having a length of 150 mm, a width of 70 mm and a thickness of 0.7 mm treated with zinc phosphate to a dry film thickness of 20 μm. It was then electrodeposited and cured by baking at 160 ° C. for 30 minutes.
[0109]
Next, the aqueous paint (initial paint) before the dilution concentration test obtained in Example 3 was applied on the electrodeposition coating film by air spray so as to have a dry film thickness of 25 μm, and baked at 150 ° C. for 30 minutes. Cured. The hardness of the obtained coating film was pencil hardness H.
[0110]
【The invention's effect】
According to the method for reusing a melamine resin-based water-based paint, the resin and the pigment do not aggregate in the water-based paint in the process of concentration, and the curability of the reused paint is reduced, or the hardness of the cured coating film and water resistance are reduced. There is no decrease in film properties such as water resistance and weather resistance.
[Brief description of the drawings]
FIG. 1 is a drawing schematically showing a configuration of a recycling system for a monochromatic water-based paint that can be used in the method of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Painting booth, 2 ... Painting gun, 4 ... Excessive spray paint, 5 ... Booth circulation water tank, 6 ... Booth circulation water, 7 ... Concentration tank, 8 ... Contents of concentration tank, 9 ... Ultrafiltration apparatus, 10 ... Filtrate tank, 11: Filtrate, 12: Recovery paint tank, 13: Concentrated paint, 20: Aqueous intermediate coating, 30: Wash tank, 31: Wash water, 50: Coating object, 70: Air flow

Claims (7)

Spraying the water-based paint on the object;
Collecting the oversprayed waterborne paint dust with booth circulating water;
Separating the collected aqueous paint and the paint diluent containing the booth circulating water into a concentrate and a filtrate using an ultrafiltration membrane;
Using the filtrate as the booth circulating water and using the concentrate as the water-based paint.
The water-based paint contains a binder component having a melamine resin as a curing agent, the booth circulating water contains a water-miscible organic solvent in an amount of 1 to 10% by weight, and is adjusted to pH 8.0 to 9.0; How to reuse water-based paint. The method according to claim 1, wherein the water-miscible organic solvent is butyl diglycol or ethylene glycol monobutyl ether. As the curing agent, a melamine resin having an outflow rate equal to or less than the maximum outflow rate theoretically calculated from the coating efficiency of the coating apparatus is selected and used. The method according to claim 1, wherein an amount of the melamine resin required for forming a coating film is secured. The maximum outflow rate is given by the equation

[Wherein, Y is the maximum outflow rate (%), and X is the coating efficiency (%) of the coating apparatus. ]
4. The method of claim 3, wherein the method is calculated by: The maximum outflow rate is given by the equation

[Where Y is the maximum outflow rate (%) and X is the coating efficiency (%). ]
4. The method of claim 3, wherein the method is calculated by: The method according to any one of claims 3 to 5, wherein the maximum outflow rate is converted into a solubility parameter, and a melamine resin having a solubility parameter equal to or less than this value is selected and used. Wherein the solubility parameter has the formula

Wherein Z is the solubility parameter and Y is the outflow rate (%). ]
7. The method of claim 6, wherein

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