JP2004066034A - Method of forming multi-layer coating film and water-base coating composition - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method which can prepare a water-base coating giving a coating film having a film performance such as low temperature curability, water resistance, and chipping resistance in a three wet coating system based on integral coating with a plastic member and form a multi-layer coating film excellent in appearance. <P>SOLUTION: In the three low temperature curing type three wet coating systems, the first water base coating used for forming the first base coating film contains an acrylic emulsion resin, a compound having an oxazoline group, a dispersant pigment-dispersed paste and a thickner, and the total amount of volatile basic substances in a unit area 1 mm<SP>2</SP>of the multi-layer coating film before being heated/cured is 7×10<SP>-6</SP>mmol or below. <P>COPYRIGHT: (C)2004,JPO

Description

【００５２】
（上記式中、Ｒ１、Ｒ２、Ｒ３およびＲ４は、各々独立に、水素原子、ハロゲン、アルキル、アラルキル、フェニルまたは置換フェニル基である。）すなわち、２−オキサゾリン基は、水素の１または２以上が置換されていてもよい。
【００５３】
上記オキサゾリン化合物（ａ）としては、たとえば、２，２’−ビス−（２−オキサゾリン）、２，２’−メチレン−ビス−（２−オキサゾリン）、２，２’−エチレン−ビス−（２−オキサゾリン）、２，２’−トリメチレン−ビス−（２−オキサゾリン）、２，２’−テトラメチレン−ビス−（２−オキサゾリン）、２、２’−ヘキサメチレン−ビス−（２−オキサゾリン）、２，２’−オクタメチレン−ビス−（２−オキサゾリン）、２，２’−エチレン−ビス−（４，４’−ジメチル−２−オキサゾリン）、２，２’−ｐ−フェニレン−ビス−（２−オキサゾリン）、２，２’−ｍ−フェニレン−ビス−（２−オキサゾリン）、２，２’−ｍ−フェニレン−ビス−（４，４’−ジメチル−２−オキサゾリン）、ビス−（２−オキサゾリニルシクロヘキサン）スルフィド、ビス−（２−オキサゾリニルノルボルナン）スルフィドなどが挙げられ、いずれか１種を単独で用いたり、２種以上を併用して用いることができる。
【００５４】
上記オキサゾリン基含有重合体（ｂ）としては、付加重合性オキサゾリンおよび必要に応じて少なくとも１種の他の重合性単量体を重合してなるものである。付加重合性オキサゾリンとしては、式（２）で表されるものである。
【００５５】
【化２】

【００５６】
（上記式中、Ｒ１、Ｒ２、Ｒ３およびＲ４は、各々独立に、水素原子、ハロゲン、アルキル、アラルキル、フェニルまたは置換フェニル基であり、Ｒ５は、付加重合性不飽和結合を持つ非環状有機基である。）
【００５７】
上記付加重合性オキサゾリンの例としては、たとえば、２−ビニル−２−オキサゾリン、２−ビニル−４−メチル−２−オキサゾリン、２−ビニル−５−メチル−２−オキサゾリン、２−イソプロペニル−２−オキサゾリン、２−イソプロペニル−４−メチル−２−オキサゾリン、２−イソプロペニル−５−エチル−２−オキサゾリンなどを挙げることができ、これらの群から選ばれる１種の化合物を単独で使用したり、または、２種以上の化合物を混合して使用したりすることができる。中でも、２−イソプロペニル−２−オキサゾリンが工業的にも入手しやすく好適である。オキサゾリン基含有化合物として、上記オキサゾリン基含有重合体（ｂ）を用いる場合に含まれる上記付加重合性オキサゾリンの使用量は特に限定されるものではないが、オキサゾリン基含有重合体（ｂ）中、１質量％以上であることが好ましい。１質量％未満の量では硬化の程度が不充分であり、耐久性、耐水性等が損なわれる傾向にある。
【００５８】
本発明に用いられる他の単量体とは、付加重合性オキサゾリンと共重合可能で、かつ、オキサゾリン基と反応しない単量体であれば、特に制限はなく、たとえば、（メタ）アクリル酸メチル、（メタ）アクリル酸ブチル、（メタ）アクリル酸２−エチルヘキシル等の（メタ）アクリル酸エステル類；（メタ）アクリロニトリル等の不飽和ニトリル類；（メタ）アクリルアミド、Ｎ−メチロール（メタ）アクリルアミド等の不飽和アミド類；酢酸ビニル、プロピオン酸ビニル等のビニルエステル類；メチルビニルエーテル、エチルビニルエーテル等のビニルエーテル類；エチレン、プロピレン等のα−オレフィン類；塩化ビニル、塩化ビニリデン、フッ化ビニル等のハロゲン化α，β−不飽和単量体類；スチレン、α−メチルスチレン等のα，β−不飽和芳香族単量体類等が挙げられ、これらの１種の化合物または２種以上の混合物を使用することができる。
【００５９】
上記オキサゾリン基含有重合体（ｂ）は付加重合性オキサゾリンおよび必要に応じて少なくとも１種の他の単量体を従来公知の重合法、たとえば懸濁重合、溶液重合、乳化重合等により製造できる。上記オキサゾリン基含有化合物の供給形態は、有機溶剤溶液、水溶液、非水ディスパーション、エマルションなど、どのような形態でもよい。
【００６０】
例えば、上記水溶性の重合体は、付加重合性オキサゾリン及び必要に応じて少なくとも１種の他の単量体を、従来公知の重合法によって水性媒体中で溶液重合を行うことにより製造できる。使用できる水性媒体は、水と混合可能なものであれば特に制限はないが、例示すれば、水；または、水とメタノール、エタノール、プロパノール、イソプロパノール、ブタノール、ターシャリーブタノール、エチレングリコール、エチレングリコールモノメチルエーテル、エチレングリコールモノブチルエーテル、ジエチレングリコール、アセトン、メチルエチルケトン等との混合溶液が挙げられる。
【００６１】
上記重合体に水溶性を付与するためには、単量体混合物中の親水性単量体の割合を５０質量％以上、好ましくは７０質量％以上で用いる。親水性単量体とは、付加重合性オキサゾリンおよびその他の単量体の中の、（メタ）アクリル酸２−ヒドロキシエチル、（メタ）アクリル酸メトキシポリエチレングリコール、（メタ）アクリル酸２−アミノエチルおよびその塩、（メタ）アクリルアミド、Ｎ−メチロール（メタ）アクリルアミド、Ｎ−（２−ヒドロキシエチル）（メタ）アクリルアミド、（メタ）アクリロニトリル、スチレンスルホン酸ナトリウム等が挙げられる。
【００６２】
本発明における第１水性ベース塗料において、上記アクリルエマルション樹脂とオキサゾリン基含有化合物との固形分質量比は、上限９９／１、下限７０／３０であることが好ましく、上限９８／２、下限８０／２０であることがより好ましい。上記比が、９９／１を超えると、硬化性が低下するおそれがあり、７０／３０未満であると、得られる塗膜の諸性能が低下するおそれがある。
【００６３】
上記アクリルエマルション樹脂及びオキサゾリン基含有化合物以外に塗膜形成性樹脂として含有することができる成分としては、揮発性の塩基性物質を上述したように、複層塗膜を形成した時点での単位面積１ｍｍ^２あたりの揮発性の塩基性物質量が７×１０^−６ｍｍｏｌ以下になるように形成できれば特に限定されず、例えば、溶剤型熱硬化性樹脂等を含有させることもできる。
【００６４】
上記溶剤型熱硬化性樹脂としては特に限定されず、例えば、アクリル樹脂；ポリエステル樹脂；アルキド樹脂；エポキシ変性、ロジン変性、フェノール樹脂変性等の変性アルキド樹脂；ポリウレタン樹脂；ポリオレフィン樹脂；ポリエーテル樹脂等の通常の溶剤型塗料に使用される樹脂を挙げることができ、これらは１種又は２種以上を使用することができる。これらのうち、顔料分散性、耐水性、耐候性の点から、アクリル樹脂、ポリエステル樹脂及びポリウレタン樹脂を好適に用いることができる。
【００６５】
上記第１水性ベース塗料は、顔料と、顔料分散剤の固形分中に揮発性の塩基性物質を含まないか、または揮発性の塩基性物質の含有量が３質量％以下である顔料分散剤とを予め分散して得られる分散剤顔料分散ペーストを含む。
【００６６】
本発明においては、上記顔料分散剤を用いることによって、第１水性ベース塗料から形成される第１ベース塗膜中の揮発性塩基性物質の量が少なくなり、得られる複層塗膜の黄変を抑えることができる。すなわち、用いた顔料分散剤が揮発性の塩基性物質を顔料分散剤の固形分に対し３質量％を超えて含有すると、得られる複層塗膜が黄変し、仕上がり外観に劣る。
【００６７】
上記揮発性の塩基性物質とは、沸点が３００℃以下の塩基性物質を意味するもので、無機及び有機の窒素含有塩基性物質を挙げることができ、具体的には、第１水性ベース塗料の記載で挙げたものである。上記塩基性物質を含有する場合には、複層塗膜形成時に、単位面積１ｍｍ^２あたりの揮発性の塩基性物質の量が７×１０^−６ｍｍｏｌ以下になるように顔料分散剤を用いることが必要である。上記アクリルエマルション樹脂の記載で述べたように上記顔料分散剤以外の成分にも、揮発性の塩基性物質が含まれる場合があることより、上記顔料分散剤に含まれる揮発性の塩基性物質量は、より少なく抑えることが好ましい。
【００６８】
すなわち、上記揮発性の塩基性物質を、実質的に含まない顔料分散剤を用いて分散することが好ましく、従来一般的に使用されているアミン中和型の顔料分散樹脂を使用しない方法が更に好ましい。
【００６９】
上記顔料分散剤は、顔料親和部分及び親水性部分を含む構造を有する樹脂である。上記顔料親和部分及び親水性部分としては、例えば、ノニオン性、カチオン性及びアニオン性の官能基を挙げることができ、顔料分散剤１分子中に上記官能基を２種類以上有していてもよい。
【００７０】
上記ノニオン性官能基としては、例えば、ヒドロキシル基、アミド基、ポリオキシアルキレン基等が挙げられ、上記カチオン性官能基としては、例えば、アミノ基、イミノ基、ヒドラジノ基等が挙げられる。また、アニオン性官能基としては、例えば、カルボキシル基、スルホン酸基、リン酸基等が挙げられる。このような顔料分散剤は、当業者にとってよく知られた方法によって製造することができる。
【００７１】
上記顔料分散剤としては、揮発性の塩基性物質を顔料分散剤の固形分中に含まないか、または３質量％以下であるものであれば特に限定されないが、少量の顔料分散剤によって効率的に顔料を分散することができるものが好ましい。
【００７２】
例えば、市販されているもの（以下いずれも商品名）を使用することもでき、具体的には、ビックケミー社製のアニオン・ノニオン系分散剤であるＤｉｓｐｅｒｂｙｋ　１９０、Ｄｉｓｐｅｒｂｙｋ　１８１、Ｄｉｓｐｅｒｂｙｋ　１８２（高分子共重合物）、Ｄｉｓｐｅｒｂｙｋ　１８４（高分子共重合物）、ＥＦＫＡ社製のアニオン・ノニオン系分散剤であるＥＦＫＡＰＯＬＹＭＥＲ４５５０、アビシア社製のノニオン系分散剤であるソルスパース２７０００、アニオン系分散剤であるソルスパース４１０００、ソルスパース５３０９５等を挙げることができる。
【００７３】
上記顔料分散剤の数平均分子量は、下限１０００、上限１０万であることが好ましい。１０００未満であると、分散安定性が充分ではない場合があり、１０万を超えると、粘度が高すぎて取り扱いが困難となる場合がある。より好ましくは、下限２０００、上限５万であり、更に好ましくは、下限４０００、上限５万である。
【００７４】
上記顔料分散剤は、顔料とともに公知の方法に従って混合分散して、分散剤顔料分散ペーストを得る。上記分散剤顔料分散ペースト製造時の配合内の上記顔料分散剤の割合は、分散剤顔料分散ペーストの固形分に対して、下限１質量％、上限２０質量％であることが好ましい。１質量％未満であると、顔料を安定に分散することができず、２０質量％を超えると、塗膜の物性に劣る場合がある。好ましくは、下限５質量％、上限１５質量％である。
【００７５】
上記顔料としては、通常の水性ベース塗料に使用される顔料であれば特に限定されないが、耐候性を向上させ、かつ隠蔽性を確保する点から、着色顔料であることが好ましい。特に二酸化チタンは着色隠蔽性に優れ、しかも安価であることから、より好ましい。
【００７６】
上記二酸化チタン以外の顔料としては、例えば、アゾキレート系顔料、不溶性アゾ系顔料、縮合アゾ系顔料、フタロシアニン系顔料、インジゴ顔料、ペリノン系顔料、ペリレン系顔料、ジオキサン系顔料、キナクリドン系顔料、イソインドリノン系顔料、ジケトピロロピロール系顔料、ベンズイミダゾロン系顔料、金属錯体顔料等の有機系着色顔料；黄鉛、黄色酸化鉄、ベンガラ、カーボンブラック等の無機着色顔料等が挙げられる。上記顔料として、更に、炭酸カルシウム、硫酸バリウム、クレー、タルク等の体質顔料を使用してもよい。
【００７７】
上記顔料としてカーボンブラックと二酸化チタンを主要顔料とした標準的なグレーの塗料として用いることもでき、上塗り塗料と明度又は色相等を合わせた塗料や各種の着色顔料を組み合わせた塗料を用いることもできる。
【００７８】
上記顔料は、第１水性ベース塗料中において、顔料及び樹脂固形分の合計質量に対する顔料の質量の比（ＰＷＣ）が、１０〜６０質量％であることが好ましい。１０質量％未満では、顔料不足のために隠蔽性が低下するおそれがある。６０質量％を超えると、顔料過多により硬化時の粘性増大を招き、フロー性が低下して塗膜外観が低下することがある。
【００７９】
上記第１水性ベース塗料中の顔料分散剤の含有量は、固形分基準で、下限０．５質量％、上限１０質量％であることが好ましい。０．５質量％未満であると、顔料分散剤の配合量が少ないために顔料の分散安定性に劣る場合がある。１０質量％を超えると、塗膜物性に劣る場合がある。好ましくは、下限１質量％、上限５質量％である。
【００８０】
上記第１水性ベース塗料は、増粘剤を含むものである。これにより、第１水性ベース塗料の粘度を高くすることができ、上記工程（１）で第１水性ベース塗料を塗装する際に、タレが発生することを抑制することができる。また、第１ベース塗膜と第２ベース塗膜との間での混層をより抑制することができる。その結果、増粘剤を含まない場合に比べて、塗装時の塗装作業性が向上し、得られる塗膜の仕上がり外観を優れたものとすることができる。
【００８１】
上記増粘剤としては特に限定されず、例えば、セルロース系のものとして、ビスコース、メチルセルロース、エチルセルロース、ヒドロキシエチルセルロース、市販されているものとしては、チローゼＭＨ及びチローゼＨ（いずれもヘキスト社製、商品名）等；アルカリ増粘型のものとして、ポリアクリル酸ナトリウム、ポリビニルアルコール、カルボキシメチルセルロース、市販されているもの（以下いずれも商品名）としては、プライマルＡＳＥ−６０、プライマルＴＴ−６１５、プライマルＲＭ−５（いずれもローム＆ハース社製）、ユーカーポリフォーブ（ユニオンカーバイト社製）等；会合型のものとして、ポリビニルアルコール、ポリエチレンオキサイド、市販されているもの（以下いずれも商品名）としては、アデカノールＵＨ−４２０、アデカノールＵＨ−４６２、アデカノールＵＨ−４７２、ＵＨ−５４０、アデカノールＵＨ−８１４Ｎ（旭電化工業社製）、プライマルＲＨ−１０２０（ローム＆ハース社製）、クラレポバール（クラレ社製）等を挙げることができる。なかでも、会合型増粘剤が好ましい。これらは、単独で用いてもよく、２種以上を併用してもよい。
【００８２】
上記増粘剤は、ウレタン会合型増粘剤であることが特に好ましい。本発明における第１水性ベース塗料中に、ウレタン会合型増粘剤が含有される場合には、水溶性溶媒中において、ウレタン会合型増粘剤中の疎水性部分同士が会合することにより、効果的に増粘作用を示すものであると推察される。これにより、セルロース系増粘剤を用いる場合に比べて、より効果的に塗料が増粘され、結果として、塗装時におけるタレを抑制することができ、また、塗料のチクソ性も向上させることができ、得られる塗膜の外観を優れたものとすることができる。
【００８３】
本発明において、上記ウレタン会合型増粘剤とは、親水性部分と疎水性部分とを両方含んでなる両親媒性増粘剤を意味し、例えば、１種又は２種以上のモノ又はポリイソシアネートと、１種又は２種以上のポリエーテルモノアルコールとを原料として反応させることによって得ることができる。
【００８４】
上記モノ又はポリイソシアネートは１分子中に１個以上のイソシアネート基を有するものであれば特に限定されず、単独で用いてもよく、２種以上を併用してもよい。
【００８５】
上記ポリエーテルモノアルコールは、分岐鎖又は２級の１価アルコールのポリエーテルであれば特に限定されない。このような化合物は、分岐鎖又は２級の１価アルコールにアルキレンオキサイド又はスチレンオキサイド等を付加重合することによって得ることができる。ここでいう分岐鎖又は２級の１価アルコールとは、特に限定されないが、例えば、炭素数の合計が下限８、上限３６であることが好ましく、下限１２、上限２４であることがより好ましい。
【００８６】
また、付加させるアルキレンオキサイドやスチレンオキサイド等は単独重合、２種類以上のランダム重合又はブロック重合によって得られるものであってもよい。重合度は、得られる塗膜の外観の観点から、下限１、上限５００であることが好ましく、下限１０、上限２００であることがより好ましい。
【００８７】
上記ウレタン会合型増粘剤を得る方法としては、例えば、モノ又はポリイソシアネートとポリエーテルモノアルコールとを、各化合物からの水酸基価とイソシアネート価との比が１．０５／１〜１．４／１となるように配合し、通常のポリエーテルとイソシアネートとの反応と同様に、例えば、８０〜９０℃で１〜３時間加熱して反応させる方法を挙げることができる。
【００８８】
上記ウレタン会合型増粘剤の市販品としては、例えば、塗装時のタレを防止する観点から、アデカノールシリーズのうち、ＵＨ−８１４Ｎ、ＵＨ−４６２、ＵＨ−４２０、ＵＨ−４７２、ＵＨ−５４０（旭電化工業社製、以上いずれも商品名）等を挙げることができる。
【００８９】
上記第１水性ベース塗料における増粘剤の含有量は、上記第１水性ベース塗料の樹脂固形分１００質量部に対して、下限０．０１質量部、上限２０質量部であることが好ましく、下限０．１質量部、上限１０質量部であることがより好ましい。０．０１質量部未満であると、増粘効果が得られず、塗装時のタレが発生するおそれがあり、２０質量部を超えると、外観及び得られる塗膜の諸性能が低下するおそれがある。
【００９０】
上記第１水性ベース塗料は、更に、紫外線吸収剤；酸化防止剤；消泡剤；表面調整剤；ワキ防止剤等の添加剤成分を添加することができる。
【００９１】
第１水性ベース塗膜形成方法
上記第１水性ベース塗料は、被塗物上に、第１ベース塗膜を形成する為に用いられる。上記被塗物（素材）としては特に限定されず、例えば、自動車に用いられるような、鋼板部材及びとプラスチック部材を被塗物とすることができる。
【００９２】
上記鋼板部材としては、例えば、鉄、アルミニウム、銅、スズ、亜鉛、ステンレス等の金属、これらを含む合金及び鋳造物等が挙げられ、プラスチック部材としては、例えば、ポリウレタン、ポリプロピレン、ポリカーボネート等の成型体及び発泡体等が挙げられる。
【００９３】
具体的に、上記鋼板部材が用いられている部位としては、ドアー、ルーフ、フード、フェンダー、トランク等が挙げられ、プラスチック部材が用いられる部位としては、バンパー、フェイシア、ミラー、モール、ガード等が挙げられる。また、自動車とは、乗用車だけでなく、トラック、オートバイ、バス、自転車等を含むものとする。
【００９４】
また、鋼板部材上に下塗り塗膜として電着塗膜を形成することができる。その場合には、カチオン型あるいはアニオン型の電着塗料を用い得る。防食性に優れた塗膜を得ることができる点より、カチオン型のものが好ましい。更に、鋼板部材は電着塗装を行う前に、予めリン酸塩、クロム酸塩等で化成処理されたものが防錆の観点から特に好ましい。
【００９５】
上記第１水性ベース塗料の塗装方法としては特に限定されず、例えば、エアースプレー；通称「リアクトガン」と言われるエアー静電スプレー；通称「マイクロ・マイクロ（μμ）ベル」、「マイクロ（μ）ベル」、「メタベル」等と言われる回転霧化式の静電塗装機等を挙げることができる。
【００９６】
上記第１水性ベース塗膜の乾燥膜厚は、用途により変化するが、５〜５０μｍであることが好ましい。上限を超えると、鮮映性が低下したり、塗装時のタレや加熱硬化時にワキ等の不具合が起こることがあり、下限を下回ると、外観が低下するおそれがある。良好な仕上がり外観を得ることを目的として、第１水性ベース塗料を塗装した後にプレヒートを行うことが好ましい。
【００９７】
工程（２）
上記工程（２）は、上述のようにして形成された第１ベース塗膜上に、第２水性ベース塗料を塗装して第２ベース塗膜を形成する工程である。
【００９８】
第２水性ベース塗料
上記第２水性ベース塗料は、主として、複層塗膜に色彩や光輝性等の美観性及び意匠性を付与し維持するために塗装されるものである。
【００９９】
本発明において、上記第２水性ベース塗料としても、顔料と、顔料分散剤の固形分中に揮発性の塩基性物質を含まないか、または３質量％以下で含む顔料分散剤とを、予め分散して得られる分散剤顔料分散ペーストが配合されているものを使用することが好ましい。すなわち、複層塗膜を形成した加熱硬化前の時点での単位面積１ｍｍ^２あたりに残存する揮発性の塩基性物質量が、７×１０^−６ｍｍｏｌ以下になるように形成できれば特に限定されない。上記揮発性の塩基性物質、及び、上記顔料分散剤としては、上記第１水性ベース塗料の記述において例示したものを挙げることができる。
【０１００】
上記第２水性ベース塗料に含まれる顔料としては、第１水性ベース塗料の記載で述べた着色顔料、体質顔料を用いることができる。例えば、レッド、ブルーあるいはブラック等の着色顔料及び／又は体質顔料を配合してソリッド型ベース塗料として用いることもできる上、更に、光輝性顔料を配合してメタリックベース塗料として用いることもできる。
【０１０１】
上記光輝性顔料としては特に限定されず、例えば、金属又は合金等の無着色若しくは着色された金属性光輝材及びその混合物、干渉マイカ粉、着色マイカ粉、ホワイトマイカ粉、グラファイト又は無色有色偏平顔料等を挙げることができる。上記光輝性顔料は、分散性に優れ、透明感及び光輝感に優れた塗膜を形成することができる。上記金属の具体例としては、アルミニウム、酸化アルミニウム、銅、亜鉛、鉄、ニッケル、スズ等を挙げることができる。
【０１０２】
上記光輝性顔料の形状は特に限定されず、更に、着色されていてもよいが、例えば平均粒径（Ｄ_５０）が、下限２μｍ、上限５０μｍであり、厚さが、下限０．１μｍ、上限５μｍである鱗片状のものが好ましい。平均粒径としては、光輝感に優れることから、より好ましくは、下限１０μｍ、上限３５μｍである。
【０１０３】
上記顔料は、１種又は２種以上を使用することができ、着色顔料及び体質顔料、並びに、必要に応じ、偏平顔料及び光輝性顔料のなかから、１種又は２種以上を組み合わせて用いることができる。
【０１０４】
上記第２水性ベース塗料において、分散剤顔料分散ペースト製造時の配合内の顔料分散剤の割合は、分散剤顔料分散ペーストの固形分に対して、３〜５０質量％であることが好ましい。３質量％未満であると、顔料を安定に分散することができず、５０質量％を超えると、得られる塗膜の物性が低下するおそれがある。
【０１０５】
上記第２水性ベース塗料は、上記分散剤顔料分散ペーストと、塗膜形成性樹脂及び硬化剤とを混合して調製することができる。
【０１０６】
上記第２水性ベース塗料に使用される塗膜形成性樹脂、硬化剤及びその他の添加剤としても特に限定されず、第１水性ベース塗料に記載したものを挙げることができる。具体的には、顔料分散性や作業性の点から、アクリル樹脂及び／又はポリエステル樹脂とメラミン樹脂との組み合わせが好ましい。また、第２水性ベース塗料の調製方法についても、上記第１水性ベース塗料において例示した方法を挙げることができる。
【０１０７】
上記光輝性顔料及びその他の全ての顔料を含めた第２水性ベース塗料中の顔料濃度（ＰＷＣ）は、一般的には、下限０．１質量％、上限５０質量％であり、好ましくは、下限０．５質量％、上限４０質量％であり、より好ましくは、下限１質量％、上限３０質量％である。５０質量％を超えると塗膜外観が低下する。
【０１０８】
上記第２水性ベース塗料中の顔料分散剤の含有量は、顔料分散剤として顔料分散剤の固形分中に揮発性の塩基性物質を含まないか、または３質量％以下で含む顔料分散剤を使用する場合には、固形分基準で、下限１質量％、上限２０質量％であることが好ましい。１質量％未満であると、顔料分散剤の配合量が少ないために顔料の分散安定性に劣る場合がある。２０質量％を超えると、得られる塗膜の物性が低下するおそれがある。
【０１０９】
第２ベース塗膜形成方法
上記第２水性ベース塗料は、上記の方法により形成された第１ベース塗膜の上に、塗装して第２ベース塗膜を形成する為に用いられる。
【０１１０】
上記塗装方法としては、第１水性ベース塗料を塗装する際に例示した方法を挙げることができる。上記第２ベース塗料を自動車車体等に対して塗装する場合には、意匠性を高めるために、エアー静電スプレーによる多ステージ塗装、好ましくは、２ステージで塗装するか、又は、エアー静電スプレーと上記の回転霧化式の静電塗装機とを組み合わせた塗装方法により行うことが好ましい。
【０１１１】
上記第２ベース塗膜の乾燥膜厚は、用途により変化するが、５〜３５μｍであることが好ましい。上限を超えると、鮮映性が低下したり、塗装時にムラ、流れ等の不具合が起こることがあり、下限を下回ると、膜切れや色ムラが発生する恐れがある。
【０１１２】
工程（３）
上記工程（３）は、上記のように形成された第２ベース塗膜の上に、更に、クリヤー塗料を塗装してクリヤー塗膜を形成する工程である。
【０１１３】
クリヤー塗料
クリヤー塗膜は、第２ベース塗料として光輝性顔料を含むメタリックベース塗料を用いた場合に光輝性顔料に起因するベース塗膜の凹凸、チカチカ等を平滑にしたり、また、第２ベース塗膜を保護し、外観を向上させるために形成されるものである。
【０１１４】
上記クリヤー塗料としては特に限定されず、例えば、塗膜形成性樹脂、硬化剤及びその他の添加剤からなるものを挙げることができる。
【０１１５】
上記塗膜形成性樹脂としては特に限定されず、例えば、アクリル樹脂、ポリエステル樹脂、エポキシ樹脂、ウレタン樹脂等が挙げられ、これらはアミノ樹脂及び／又はポリイソシアネート樹脂等の硬化剤と組み合わせて用いることができる。透明性又は耐酸エッチング性等の点から、アクリル樹脂及び／若しくはポリエステル樹脂とアミノ樹脂及び／又はポリイソシアネート樹脂との組み合わせ、又は、カルボン酸・エポキシ基硬化系を有するアクリル樹脂及び／若しくはポリエステル樹脂等を用いることが好ましい。
【０１１６】
上記クリヤー塗料としては、上述した第２ベース塗料を塗装後、ウエット塗膜の状態で塗装するため、塗膜層間のなじみや反転、又は、タレ等の防止のため、粘性制御剤を添加剤として含有することが好ましい。上記粘性制御剤の添加量は、クリヤー塗料の樹脂固形分１００質量部に対して、下限０．０１質量部、上限１０質量部であり、好ましくは、下限０．０２質量部、上限８質量部、より好ましくは、下限０．０３質量部、上限６質量部である。１０質量部を超えると、得られた塗膜の外観が低下し、０．１質量部未満であると、粘性制御効果が得られず、塗膜形成時にタレ等の不具合を起こす原因となる。
【０１１７】
上記クリヤー塗料の塗料形態としては、有機溶剤型、水性型（水溶性、水分散性、エマルジョン）、非水分散型、粉体型のいずれでもよく、また必要により、硬化触媒、表面調整剤等を用いることができる。
【０１１８】
クリヤー塗膜形成方法
上記クリヤー塗料の調製方法及び塗装方法としては、従来の方法に従って行うことができる。上記クリヤー塗膜の乾燥膜厚は、用途により変化するが、１０〜７０μｍである。この乾燥膜厚が上限を超えると、鮮映性が低下したり、塗装時にムラ、流れ等の不具合が起こることがあり、下限を下回ると、得られた塗膜の外観が低下する恐れがある。
【０１１９】
工程（４）
上記工程（４）においては、上記第１ベース塗膜、上記第２ベース塗膜及び上記クリヤー塗膜を同時に加熱硬化させて、複層塗膜を得る。
【０１２０】
上記加熱硬化させる温度としては、下限８０℃、上限１６０℃、好ましくは、下限９０℃、上限１４０℃にて行うことによって、高い架橋度の硬化塗膜を得ることができる。１６０℃を超えると、塗膜が固く脆くなり、８０℃未満では硬化が充分ではない。硬化時間は硬化温度により変化するが、９０〜１４０℃で１０〜６０分間が適当である。
【０１２１】
本発明の複層塗膜形成方法においては、上記工程（１）、上記工程（２）及び上記工程（３）により形成された複層塗膜が工程（４）を行う前に、単位面積１ｍｍ^２における揮発性の塩基性物質の合計量が７×１０^−６ｍｍｏｌ以下である。
【０１２２】
上記工程（１）、（２）及び（３）の塗装により得られた複層塗膜には、上記第１水性ベース塗料及び上記第２水性ベース塗料に含まれる揮発性の塩基性物質が残存する。この残存した揮発性の塩基性物質の合計量が塗膜の単位面積１ｍｍ^２において７×１０^−６ｍｍｏｌを超える場合に、上記複層塗膜を硬化させるべく加熱すると、上記揮発性の塩基性物質が残存することに起因して、化学変化を起こして黄変を来す。また、第１ベース塗膜や第２ベース塗膜から揮散する塩基性物質がクリヤー塗膜の内部に捕捉され、上記クリヤー塗膜においても黄変を来すため、色再現性や意匠性が低下する。好ましくは、６．５×１０^−６ｍｍｏｌ以下である。
【０１２３】
本明細書において、上記単位面積１ｍｍ^２における上記揮発性の塩基性物質の合計量（ｍｍｏｌ）とは、上記工程（１）、（２）及び（３）により形成された複層塗膜の表面上の１ｍｍ^２の面積を有する区画（Ｓ）を上記被塗装物の塗装面に垂直に投影して得られる区画（Ｓ′）を仮定した場合に、上記区画（Ｓ）及び上記区画（Ｓ′）によって挟まれた部分（Ｖ）に含有される総揮発性塩基性物質の含有量（ｍｍｏｌ）を意味する。
【０１２４】
上記単位面積１ｍｍ^２における上記揮発性塩基性物質の合計量（ｍｍｏｌ）は、上記部分（Ｖ）における第１ベース塗膜、第２ベース塗膜及びクリヤー塗膜の各塗膜が含有する揮発性の塩基性物質の量（ｍｍｏｌ）の合計量となる。即ち、上記部分（Ｖ）における上記第１ベース塗膜が含有する揮発性の塩基性物質の量〔Ａｍｍｏｌ〕と、上記部分（Ｖ）における上記第２ベース塗膜が含有する揮発性の塩基性物質の量〔Ｂｍｍｏｌ〕と、上記部分（Ｖ）における上記クリヤー塗膜が含有する揮発性の塩基性物質の量〔Ｃｍｍｏｌ〕との合計量〔（Ａ＋Ｂ＋Ｃ）ｍｍｏｌ〕が、本明細書において塗膜単位面積１ｍｍ^２における上記揮発性の塩基性物質の合計量となる。
【０１２５】
上記単位面積１ｍｍ^２における上記揮発性の塩基性物質の合計量（ｍｍｏｌ）は、上記工程（１）、（２）及び（３）により形成された複層塗膜のサンプルを採取し、採取したサンプルについてガスクロマトグラフィーで定量した上記揮発性の塩基性物質の含有量と、乾燥膜厚とから、１ｇを１ｃｍ^３であると仮定して算出することにより求めることができる。
【０１２６】
本発明の塗膜形成方法によって得られる複層塗膜の膜厚は、通常、下限３０μｍ、上限３００μｍ、好ましくは、下限５０μｍ、上限２５０μｍである。３００μｍを超えると、冷熱サイクル等の膜物性が低下し、３０μｍ未満であると、膜自体の強度が低下する。
【０１２７】
本発明の第１水性ベース塗料は、上記複層塗膜形成方法における第１ベース塗膜を形成する為に用いる塗料であって、上記第１水性ベース塗料は、アクリルエマルション樹脂、オキサゾリン基含有化合物、分散剤顔料分散ペースト及び増粘剤を含有するものである。上記複層塗膜形成方法において、第１水性ベース塗料を用いることにより、優れた耐水性、耐溶剤性、耐チッピング性等の塗膜性能及び塗装時のタレや塗膜の黄変を防止することができ、優れた外観の塗膜を得ることができる。
【０１２８】
また、本発明の複層塗膜形成方法においては、上記第１水性ベース塗料がアクリルエマルション樹脂、オキサゾリン基含有化合物、分散剤顔料分散ペースト及び増粘剤を含有するものであり、上記工程（１）、工程（２）及び工程（３）により形成された塗膜が、工程（４）を行う前に、単位面積１ｍｍ^２における揮発性の塩基性物質の合計量が７×１０^−６ｍｍｏｌ以下であることから、塗装時のタレや塗膜の黄変を抑制することができ、優れた外観の複層塗膜を得ることができる。特に、増粘剤としてウレタン会合型増粘剤を用いる場合には、塗装時のタレをより効果的に抑制することができることからとから、より優れた仕上がり外観の塗膜を得ることができる。
【０１２９】
更に、本発明の３ウエット塗装方法により、従来一般的であった３コート２ベーク法におけるよりも、ベース塗膜の加熱硬化工程を省くことができるので、工程短縮、コスト削減、エネルギー消費量削減及び環境負荷低減を目指す新規塗装システムを構築することができる。
【０１３０】
【実施例】
以下に本発明を実施例により更に具体的に説明するが、本発明はこれら実施例のみに限定されるものではない。また実施例中、「部」は特に断りのない限り「質量部」を意味する。
【０１３１】
製造例１　着色顔料ペーストの製造
市販分散剤「Ｄｉｓｐｅｒｂｙｋ　１９０」（ビックケミー社製ノニオン・アニオン系分散剤、商品名）９．４部、イオン交換水３６．８部、ルチル型二酸化チタン３４．５部、硫酸バリウム３４．４部及びタルク６部を予備混合を行った後、ペイントコンディショナー中でガラスビーズ媒体を加え、室温で粒度５μｍ以下となるまで混合分散し、分散剤着色顔料分散ペーストを得た。
【０１３２】
製造例２　アクリルエマルション樹脂の製造
反応容器に脱イオン水３５．７５部を加え、窒素気流中で混合攪拌しながら８０℃に昇温した。次いでα、β−エチレン性不飽和モノマー混合物として、スチレン１０部、メタクリル酸メチル２４．０２部、アクリル酸ブチル２８．９４部、アクリル酸エチル２０．１１部、アクリル酸−４ヒドロキシブチル１５．４０部、メタクリル酸１．５３部、アクアロンＨＳ−１０（ポリオキシエチレンアルキルプロペニルフェニルエーテル硫酸エステル、第一工業製薬社製アニオン系分散剤、商品名）３．０部、アデカリアソープＮＥ−２０（旭電化工業社製ノニオン系分散剤、商品名）０．５部及び脱イオン水５０部からなるモノマー乳化物、過流酸アンモニウム０．３０部及び脱イオン水１５．０部からなる開始剤溶液を２時間にわたり並行して反応容器に滴下した。滴下終了後、同温度で２時間熟成を行った。次いで、４０℃まで冷却し、４００メッシュフィルターでろ過し、粒径２００ｎｍ、不揮発分５０％、酸価１０ｍｇＫＯＨ／ｇ、水酸基価６０のアクリルエマルション樹脂を得た。
【０１３３】
実施例１
（１）第１水性ベース塗料１の調製
表１に示すように、製造例１で得られた分散剤着色顔料分散ペーストを６０．３部、製造例２で得られたアクリルエマルション樹脂を１１１．４部、エポクロスＷＳ−５００（日本触媒社製オキサゾリン基含有化合物、水溶性型アクリル共重合体、オキサゾリン価２２０［ｓｏｌｉｄ／ｅｑ］、商品名）を５．５部混合した後、アデカノールＵＨ−８１４Ｎ（ウレタン会合型増粘剤、有効成分３０％、旭電化工業社製、商品名）１．０部を混合攪拌し、第１水性ベース塗料１を得た。
【０１３４】
【表１】

【０１３５】
（２）塗膜形成方法
リン酸亜鉛処理したダル鋼板に、パワートップＵ−５０（日本ペイント社製カチオン電着塗料、商品名）を、乾燥塗膜が２０μｍとなるように電着塗装し、１６０℃で３０分間の加熱硬化後冷却して、鋼板基板を得た。また、予め脱脂した自動車バンパー用ポリプロピレン素材からなる１０×２０ｃｍのテストピースをプラスチック部材として準備した。
【０１３６】
得られた両基板に、上記第１水性ベース塗料１をエアースプレー塗装にて２０μｍ塗装し、８０℃で５分プレヒートを行った後、アクアレックスＡＲ−２０００シルバーメタリック（日本ペイント社製水性メタリックベース塗料、商品名）をエアースプレー塗装にて１０μｍ塗装し、８０℃で３分プレヒートを行った。更に、その塗板にクリヤー塗料として、Ｒ２９８クリヤー（日本ビーケミカル社製イソシアネート硬化型クリヤー塗料、商品名）をエアースプレー塗装にて３５μｍ塗装した後、１００℃で３０分間の加熱硬化を行い試験片を得た。
【０１３７】
また、硬化前の複層塗膜に残存する塩基性物質量を後述した方法により測定すると、４．４ｍｍｏｌであった。更に、加熱硬化後に得られた複層塗膜は、仕上がり外観に優れ、塗膜の黄変性という観点においても、全く変化無く良好なものであった。
【０１３８】
なお、上記第１水性ベース塗料（第１水性ベース塗料１）、第２水性ベース塗料（アクアレックスＡＲ−２０００シルバーメタリック）、クリヤー塗料（Ｒ２９８クリヤー）は、下記条件で希釈し、塗装に用いた。
【０１３９】
（第１水性ベース塗料）
シンナー：イオン交換水
４０秒／ＮＯ．４フォードカップ／２０℃
（第２水性ベース塗料）
シンナー：イオン交換水
４５秒／ＮＯ．４フォードカップ／２０℃
（クリヤー塗料）
シンナー：ＥＥＰ（エトキシエチルプロピオネート）／Ｓ−１５０（エクソン社製芳香族系炭化水素溶剤、商品名）＝１／１
３０秒／ＮＯ．４フォードカップ／２０℃
【０１４０】
（３）塗膜の評価
〔耐水試験〕
得られた鋼板塗板を４０℃の温水に１０日間浸積し、洗浄１時間後の外観を目視により観察し、下記の基準により評価した。
外観評価基準
５；変化無し
４；温水境界部が、かすかに膨潤する
３；温水境界部が、かすかに黒ずんでいる
２；温水境界部が、黒ずんでいる
１；温水境界部が膨潤し、塗膜が黒ずんでいる
【０１４１】
〔耐チッピング性〕
得られた鋼板塗板の耐チッピング性の評価を以下のようにして行った。グラベロテスター試験機（スガ試験機社製）を用いて、７号砕石３００個を３５ｃｍの距離から３．０ｋｇｆ／ｃｍ^２の空気圧で、塗膜に４５°の角度で衝突させた。水洗乾燥後、ニチバン社製工業用ガムテープを用いて剥離テストを行い、その後、塗膜のはがれの程度を、目視により観察し評価した。
判断基準
５；全く剥離なし
４；剥離面積が小さく、頻度も少ない
３；剥離面積は小さいが、頻度がやや多い
２；剥離面積は大きいが、頻度は少ない
１；剥離面積が大きく、頻度も多い
【０１４２】
〔塩基性物質量測定方法〕
第１ベース塗膜及び第２ベース塗膜のサンプルを、それぞれ上記塗装方法において第１水性ベース塗料、第２水性ベース塗料を塗装しプレヒートした後、採取した。採取したサンプルを０．３ｇ秤量し、標準溶液（メタノール１００ｍｌにイソブタノール１．０ｇ混合）を０．６ｍｌ加えた後、タッチミキサー、超音波振動で充分に攪拌し、更に遠心分離した後、上澄み液を採取し、ガスクロマトグラフィーで塩基性物質量を定量した。得られた塩基性物質量と乾燥膜厚とから、１ｇを１ｃｍ^３であると仮定して、塗膜の単位面積１ｍｍ^２における塩基性物質量の合計量を算出した。
【０１４３】
〔付着性評価〕
得られたポリプロピレン塗板を、カッターナイフを用いて、一片が２ｍｍの碁盤目状に、素地に達する程度に１００個の切り目を設け、その表面にニチバン社製セロファンテープを親指で圧着した。その後、上方４５度方向に勢い良く引き剥がして、塗膜の各部材への付着性を試験した。結果を下記の判断基準に従い判断した。
付着性評価基準
５；全く剥がれない
４；１個以上１０個未満の剥がれ
３；１０個以上５０個未満の剥がれ
２；５０個以上９０個未満の剥がれ
１；９０以上の剥がれ
以上の評価結果を表２に示した。
【０１４４】
実施例２〜４
下記表２に示す塗料の組み合わせ及び硬化方法で複層塗膜を作成した以外は、実施例１と同様にして試験片を得た。尚、実施例４では、クリヤー塗料として、マックフロー　Ｏ−１８００Ｗ−２クリヤー（日本ペイント社製酸エポキシ硬化型クリヤー塗料、商品名）を用いた。また、同様に評価し、結果を表２に示した。
【０１４５】
実施例５
下記に示す方法で第１水性ベース塗料を調製し、クリヤー塗料として、マックフロー　Ｏ−１８００Ｗ−２クリヤー（日本ペイント社製酸エポキシ硬化型クリヤー塗料、商品名）を用いた以外は、実施例１と同様にして試験片を得た。また、同様に評価し、結果を表２に示した。
【０１４６】
（１）第１水性ベース塗料２の調製
表１に示すように、製造例１で得られた分散剤着色顔料分散ペーストを６０．３部、製造例２で得られたアクリルエマルション樹脂を１１１．４部、オキサゾリン基含有化合物として２，２’−ｍ−フェニレン−ビス−（２−オキサゾリン）を１．０部混合した後、アデカノールＵＨ−８１４Ｎ（ウレタン会合型増粘剤、有効成分３０％、旭電化工業社製、商品名）１．０部混合攪拌し、第１水性ベース塗料２を得た。
【０１４７】
比較例１
下記に示す方法で第１水性ベース塗料を調製した以外は、実施例１と同様にして試験片を得た。また、同様に評価し、結果を表２に示した。
【０１４８】
（１）第１水性ベース塗料３の調製
表１に示すように、製造例１で得られた分散剤着色顔料分散ペーストを６０．３部、製造例２で得られたアクリルエマルション樹脂を８１．１部、サイメル３２７（三井サイテック社製イミノ型メラミン樹脂、商品名）１９．３部を混合攪拌し、第１水性ベース塗料３を得た。
【０１４９】
【表２】

【０１５０】
実施例で得られた第１水性ベース塗料は、比較例で得られたものに比べて、耐水性及び耐チッピング性において優れた値を示すものであり、特に低温硬化時での性能の低下が全くなかった。また、ポリプロピレン部材に対する付着性においても良好であり、揮発性の塩基性物質の含有量を抑制したことにより黄変等の異常も見られず、得られた複層塗膜は仕上がり外観に優れるものであった。
【０１５１】
【発明の効果】
本発明の複層塗膜形成方法は、被塗物上に、第１水性ベース塗料、第２水性ベース塗料及びクリヤー塗料を用いて複層塗膜を形成し、同時に加熱硬化させる工程からなる複層塗膜形成方法であり、特に残存塩基性物質量が少ない水性塗料を用いることより、３ウェット１ベーク塗装システムにおいても塗膜の黄変を抑制することができ、耐水性、耐チッピング性、低温硬化性、ポリプロピレン部材に対する付着性及び優れた外観を有する塗膜を形成することができた。
【０１５２】
すなわち、本発明の複層塗膜形成方法は、塗装工程短縮、コスト削減及び環境負荷低減を目指す３ウエット塗装システムにおいて、特に自動車車体等の車両塗装に、好適に使用することができ、更に低温硬化性が高いことより、プラスチック部材との一体塗装を踏まえた水性ベース塗料組成物を提供することができる。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for forming a multilayer coating film and an aqueous base coating composition.
[0002]
[Prior art]
In recent years, in order to reduce the number of heat-curing steps, save resources and save energy as automotive painting finishing procedures, three painting steps of base coating and clear coating as intermediate coating and top coating on the object to be coated have been carried out. Attempts have been made to employ a three-wet coating system which is carried out by a wet-on-wet coating method and simultaneously cures the resulting coating.
[0003]
Further, from the viewpoint of color matching between members such as an automobile body, saving space and energy saving in a painting process, an attempt has been made to integrally paint a steel plate member used for an automobile body and a so-called plastic member. However, in consideration of the thermal deformation of the plastic member and the like, it is not possible to heat and cure at a high temperature as in the case where a steel plate member is conventionally processed. Therefore, development of a paint that can be cured at a low temperature is desired.
[0004]
On the other hand, water-based paints have attracted attention for reducing environmental load (VOC and the like). In general, water-based paints are prepared by combining a film-forming resin incorporating a hydrophilic functional group such as an anionic functional group with a counter ion to make the resin water-soluble and water-dispersible in a diluting solvent such as water. Or contains an emulsion.
[0005]
For example, when an anionic functional group such as a carboxyl group, a sulfonic acid group, a sulfate group, or a phosphate group is introduced as the hydrophilic functional group, a basic ion such as an amine is used as a counter ion thereof. Although a substance is used, it is said that the basic substance causes a chemical change upon heating or forms a color-forming substance to cause yellowing of the coating film.
[0006]
However, since basic substances such as amines cause alkali swelling of the film-forming resin and impart viscosity to the coating material, reducing the content of the basic material decreases coating workability and lowers the finished appearance. was there.
[0007]
[Problems to be solved by the invention]
In view of the above situation, the present invention is an aqueous base paint excellent in low-temperature curability based on integral coating with a plastic member, which is applicable to a three-wet coating system aiming at shortening the coating process, saving energy, and thus reducing the environmental burden. And a method capable of forming a multilayer coating film having excellent coating performance such as water resistance and chipping resistance and a high appearance by using the same, and an aqueous base coating composition realizing the method. It is intended to provide.
[0008]
[Means for Solving the Problems]
The present invention provides a step (1) of applying a first aqueous base paint on an object to be coated to form a first base coat, and applying a second aqueous base paint on the first base coat. A step (2) of forming a second base coating film, further a step (3) of applying a clear paint thereon to form a clear coating film, and the first base coating film and the second base coating film A method for forming a multilayer coating film comprising a step (4) of simultaneously heating and curing a film and the clear coating film to obtain a multilayer coating film, wherein the first aqueous base coating material comprises an acrylic emulsion resin and an oxazoline group-containing resin. It contains a compound, a dispersant, a pigment-dispersed paste and a thickener, and the coating film formed by the step (1), the step (2) and the step (3) is converted into a unit before the step (4). Area 1mm²The total amount of volatile basic substances is 7 × 10^-6It is intended to provide a method for forming a multilayer coating film, characterized in that the amount is less than or equal to mmol.
[0009]
Further, the present invention is an aqueous base paint used in the method for forming a multilayer coating film, wherein the first aqueous base paint contains an acrylic emulsion resin, an oxazoline group-containing compound, a dispersant pigment dispersion paste, and a thickener. The present invention provides an aqueous base coating composition characterized by the above. Hereinafter, the present invention will be described in detail.
[0010]
BEST MODE FOR CARRYING OUT THE INVENTION
In the method for forming a multilayer coating film of the present invention, a step (1) of applying a first aqueous base coating material on an object to be coated to form a first base coating film; A step (2) of applying a water-based base paint to form a second base coat, a step (3) of applying a clear paint thereon to form a clear coat, and the first base coat Step (4) of simultaneously heating and curing the film, the second base coating film, and the clear coating film to obtain a multilayer coating film.
[0011]
In the present invention, the first aqueous base paint, the second aqueous base paint, and the clear paint can be applied in this order by a wet-on-wet coating method.
[0012]
When the coating is performed by the wet-on-wet coating method, a preheating step is included. However, the preheating means applying heat or warm air to the coating film for the main purpose of evaporating water remaining in the coating film after coating.For example, the heat applied to the coating film is from room temperature to It means leaving or heating at 100 ° C. for 1 to 10 minutes.
[0013]
Process (1)
In the method for forming a multilayer coating film of the present invention, the step (1) is a step of applying a first aqueous base coating material on an object to be coated to form a first base coating film.
[0014]
First water base paint
The first aqueous base paint used for forming the first base coating film covers the base, secures the surface smoothness after top coating (improves the appearance), and has coating properties such as adhesion, impact resistance, and chipping resistance. It is painted to give film properties.
[0015]
The first aqueous base paint contains an acrylic emulsion resin, an oxazoline group-containing compound, a dispersant pigment dispersion paste, and a thickener.
[0016]
The acrylic emulsion resin can be obtained as an aqueous dispersion by one or more methods of introducing an anionic functional group into the resin, using an anionic dispersant, a nonionic dispersant or a cationic dispersant. it can.
[0017]
In particular, when the above-described anionic functional group is introduced into the resin or when an anionic dispersant is used, it is considered necessary to use a basic substance such as an amine as a counter ion of the anionic functional group. In this case, the amount of the basic substance used is reduced as much as possible, or a method of introducing the anionic functional group into the resin, a method of using an anionic dispersant, a nonionic dispersant or a cationic dispersant. It is preferable to disperse in water by a method in which a method using an agent is used in combination.
[0018]
Since the basic substance is said to undergo a chemical change or form a color-forming substance upon heating to turn into a coating yellowing substance, the acrylic emulsion resin dispersed in water by the above method is used to apply the basic substance. Yellowing of the film can be suppressed, and a coating film having an excellent appearance can be obtained.
[0019]
Furthermore, the method of introducing the anionic functional group into the resin, and the amount of the basic substance used in combination when using an anionic dispersant, at the time before heat curing to form a multilayer coating film, Unit area 1mm²The amount of volatile basic substance remaining around^-6It is necessary to form a coating film so that the amount becomes less than mmol. In addition to the first aqueous base paint, a volatile basic substance may be contained in a second aqueous base paint to be described later. Therefore, the amount of the volatile basic substance contained in the first aqueous base paint is , It is preferable to press less.
[0020]
In particular, when the upper coating film such as a clear coating film is applied over again, the basic substance which is volatilized is trapped inside the upper coating film, and yellowing becomes remarkable in the upper coating film. Decreases. The method for measuring the amount of volatile basic substance in the first base coating film is to be performed in accordance with the description in step (4) described later.
[0021]
The acrylic emulsion resin preferably has an acid value of resin solid content of a lower limit of 1 mgKOH / g and an upper limit of 100 mgKOH / g. When the amount is less than 1 mgKOH / g, the adhesiveness is poor when formed into a coating film, and poor curing may occur. When the amount exceeds 100 mgKOH / g, the hydrophilicity is too high and the water resistance of the coating film deteriorates. Sometimes. More preferably, the lower limit is 3 mgKOH / g and the upper limit is 70 mgKOH / g.
[0022]
When the acrylic emulsion resin contains a hydroxyl group, the hydroxyl value of the resin solid content preferably has a lower limit of 30 and an upper limit of 150. If it is less than 30, curing failure may occur. If it exceeds 150, excessive hydroxyl groups may remain in the cured coating film, resulting in poor water resistance. More preferably, the lower limit is 40 and the upper limit is 120.
[0023]
The method for producing the acrylic emulsion resin is not particularly limited and can be appropriately selected depending on the type and properties of the intended acrylic emulsion resin, and includes, for example, conventionally known methods such as an interfacial polymerization method and a solution polymerization method. Manufacturing methods can be used.
[0024]
The particle size of the acrylic emulsion resin is preferably in the range of 0.01 to 0.5 μm. When the particle size is less than 0.01 μm, there are many problems in production, and when it is more than 0.5 μm, the stability of the coating material is reduced and the appearance of the obtained coating film is deteriorated. The particle size can be adjusted by a method well known to those skilled in the art, for example, by changing the type and amount of the surfactant or the monomer composition.
[0025]
Representative polymerizable monomers having a functional group used for synthesizing the acrylic emulsion resin are as follows. Examples of those having a hydroxyl group include, for example, hydroxyethyl (meth) acrylate, hydroxypropyl (meth) acrylate, hydroxybutyl (meth) acrylate, hydroxymethyl methacrylate, allyl alcohol, hydroxyethyl (meth) methacrylate and ε- And adducts with caprolactone.
[0026]
On the other hand, those having an acidic group include polymerizable monomers having a carboxyl group, a sulfonic acid group, and the like. Examples of those having a carboxyl group include (meth) acrylic acid, crotonic acid, ethacrylic acid, propylacrylic acid, isopropylacrylic acid, itaconic acid, maleic anhydride, fumaric acid, and the like. Examples of the polymerizable monomer having a sulfonic acid group include t-butylacrylamide sulfonic acid. When a polymerizable monomer having an acidic group is used, a part of the acidic group is preferably a carboxyl group.
[0027]
In addition, glycidyl group-containing unsaturated monomers such as glycidyl (meth) acrylate, isocyanate group-containing unsaturated monomers such as m-isopropenyl-α, α-dimethylbenzyl isocyanate and isocyanatoethyl acrylate are functionalized. It can be mentioned as a polymerizable monomer having a group.
[0028]
Other polymerizable monomers include, for example, methyl (meth) acrylate, ethyl (meth) acrylate, isopropyl (meth) acrylate, n-propyl (meth) acrylate, and n- (meth) acrylate. Butyl, t-butyl (meth) acrylate, isobutyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, n-octyl (meth) acrylate, lauryl (meth) acrylate, stearyl (meth) acrylate, Alkyl (meth) acrylates such as tridecyl methacrylate; addition products of oil fatty acids with acrylic acid or methacrylate monomers having an oxirane structure (for example, addition products of stearic acid and glycidyl methacrylate);₃An addition reaction product of the above oxirane compound containing an alkyl group with acrylic acid or methacrylic acid, styrene, α-methylstyrene, o-methylstyrene, m-methylstyrene, p-methylstyrene, pt-butylstyrene, ( Benzyl meth) acrylate, itaconic acid ester (dimethyl itaconate, etc.), maleic acid ester (dimethyl dimethyl maleate, etc.), fumaric acid ester (dimethyl fumarate, etc.), acrylonitrile, methacrylonitrile, methyl isopropenyl ketone, Vinyl acetate, Beoba monomer (trade name, manufactured by Shell Chemical Co., Ltd.), vinyl propionate, vinyl pivalate, vinyl propionate, ethylene, propylene, butadiene, N, N-dimethylaminoethyl acrylate, N, N-dimethylaminoethyl methacrylate And polymerizable monomers such as acrylic acid, acrylamide, and vinylpyridine.
[0029]
The polymerization reaction for obtaining the acrylic emulsion resin is preferably performed in the presence of a radical polymerization initiator. Examples of the radical polymerization initiator include azo initiators such as 2,2′-azobisisobutyronitrile and 2,2′-azobis (2,4-dimethylvaleronitrile), benzoyl peroxide, lauryl peroxide, and the like. t-butyl peroctoate and the like. The use amount of these initiators is 0.2 to 10 parts by mass, preferably 0.5 to 5 parts by mass, per 100 parts by mass of the total of the polymerizable monomers.
[0030]
In the present invention, the anionic functional group in the case where an anionic functional group is introduced into the acrylic emulsion resin and dispersed in water is not particularly limited, and a carboxyl group, a sulfonic acid group, a sulfate group, a phosphate group, or the like is used. And the like. In the case of dispersing in water using an anionic dispersant as a surfactant, use a dispersant having an anionic functional group such as a carboxyl group, a sulfonic acid group, a sulfate group, or a phosphate group as a hydrophilic group. be able to.
[0031]
In addition, examples of the counter ion used when the acrylic resin is formed into an aqueous dispersion using the anionic functional group include a volatile basic substance. This volatile basic substance means a basic substance having a boiling point of 300 ° C. or lower, and examples thereof include inorganic and organic nitrogen-containing basic substances.
[0032]
Among the volatile basic substances, examples of the inorganic basic substance include ammonia, and examples of the organic basic substance include methylamine, dimethylamine, trimethylamine, ethylamine, diethylamine, triethylamine, and isopropyl. C1-C20 linear or branched alkyl group-containing tertiary amines such as amine, diisopropylamine and dimethyldodecylamine; carbon such as monoethanolamine, diethanolamine and 2-amino-2-methylpropanol A linear or branched hydroxyalkyl group-containing tertiary amine having 1 to 20 carbon atoms; a linear or branched alkyl group having 1 to 20 carbon atoms such as dimethylethanolamine, diethylethanolamine and the like; Contain 1 to 20 linear or branched hydroxyalkyl groups C1-C20 substituted or unsubstituted chain polyamines such as diethylenetriamine and triethylenetetramine; C1-C20 substituted or unsubstituted cyclics such as morpholine, N-methylmorpholine and N-ethylmorpholine Monoamines include amines such as substituted or unsubstituted cyclic polyamines having 1 to 20 carbon atoms such as piperazine, N-methylpiperazine, N-ethylpiperazine, N, N-dimethylpiperazine.
[0033]
Furthermore, as the anionic dispersant, a portion having an affinity for the film-forming resin component and a structure having the anionic functional group as a hydrophilic group may be used. They need not be distinguished.
[0034]
For example, commercially available anionic surfactants can be used, and specifically (all of which are trade names below), Aqualon HS-10 (manufactured by Daiichi Kogyo Seiyaku Co., Ltd.), Adecaria Soap SE-10N (Asahi Denka) Industrial Co., Ltd.), Adecaria Soap SE-20N (Asahi Denka Kogyo Co., Ltd.), Newcol 560-SN (Nippon Emulsifier Co., Ltd.), Revenol WZ (Kao Co., Ltd.), Perex SS-H (Kao Co., Ltd.), etc. Can be mentioned.
[0035]
Further, in order to form an aqueous dispersion of the acrylic emulsion resin used in the present invention, when using a nonionic dispersant having a nonionic functional group, a volatile basic substance in the coating film as a counter ion Absent, no yellowing of the coating.
[0036]
The nonionic dispersant is not particularly limited as long as it has a structure containing a portion having an affinity for the film-forming resin component and a hydrophilic group, and the surfactant and the resin to be dispersed are strictly defined. They need not be distinguished. The nonionic dispersant has a nonionic hydrophilic group. However, in the present invention, a nonionic dispersant having a cationic functional group in the molecule is not excluded.
[0037]
The nonionic dispersant is not particularly limited, and examples thereof include polyoxyalkylene alkyl ethers such as polyoxyethylene lauryl ether, polyoxyethylene oleyl ether, and polyoxyethylene stearyl ether; polyoxyethylene alkyl phenyl ether; polyoxyethylene sorbitan Polyoxyalkylene ethers and / or fatty acid esters of sorbitan such as stearates; oxyalkylene copolymers such as oxyethyleneoxypropylene block copolymers; glycerin polyoxyalkylene ethers and / or fatty acid esters such as polyoxyethylene glyceride; Can be mentioned. Among them, polyoxyalkylene alkyl ethers are preferable, and polyoxyethylene lauryl ether, polyoxyethylene oleyl ether, and polyoxyethylene stearyl ether are more preferable.
[0038]
For example, as the nonionic dispersant, a commercially available nonionic surfactant can be used. Specifically, Newcol 1120, Newcol 1820 (manufactured by Nippon Emulsifier Co., Ltd., all of which are trade names), Adecaria Soap NE-20 (trade name, manufactured by Asahi Denka Kogyo KK) and the like.
[0039]
As the nonionic dispersing agent, a nonionic dispersing resin in which a nonionic functional group such as a hydroxyl group, an oxyethylene group, or an amide group is introduced into the resin can also be used.
[0040]
Further, when a cationic dispersant having a cationic functional group is used, -COO present as a counter ion of the cationic functional group is used.⁻Anions having an acidic group do not cause yellowing of the coating film.
[0041]
The cationic dispersant is not particularly limited as long as it has a portion having an affinity for the film-forming resin component and a structure containing a hydrophilic group, and the surfactant and the resin to be dispersed are strictly defined. They need not be distinguished. The cationic dispersant has a cationic functional group that is ionized in water to become an organic cation.However, in the present invention, a dispersant having a nonionic functional group in the molecule is not excluded. .
[0042]
The cationic dispersant may have a counter ion. Examples of the counter ion include inorganic acids such as hydrochloric acid, nitric acid, and phosphoric acid; formic acid, acetic acid, lactic acid, sulfamic acid, and acetylglycinic acid. And those derived from organic acids.
[0043]
The cationic dispersant is not particularly restricted but includes, for example, alkyltrimethylammonium chloride; polyoxyethylenealkylamine and the like. Commercially available cationic surfactants can also be used as the cationic dispersant. Examples of the commercially available products include ADEKAMIN 4MAC-30 and ADEKAMIN LDM (manufactured by Asahi Denka Co., Ltd., all of which are trade names). Can be mentioned.
[0044]
As the cationic dispersant, for example, a cationic dispersing resin into which a cationic functional group such as an amino group, an imino group, or a hydrazino group has been introduced can also be used.
[0045]
When a surfactant is used as the anionic, nonionic or cationic dispersant, the hydrophilic-lipophilic balance (HLB) is preferably 6 or more. If it is less than 6, the hydrophilicity may be too low and the resin component may not be sufficiently dispersed. More preferably, it is 12 or more.
[0046]
The number average molecular weight of the dispersant is preferably from 400 to 20,000. If it is less than 400, the dispersion of the resin component may be insufficient, and if it exceeds 20,000, handling of the obtained resin may be difficult. More preferably, it is 600 to 10,000. One or more of the above dispersants can be used, respectively, and these may be used in combination.
[0047]
The content of the dispersant is preferably 0.5 to 20% by mass based on the solid content based on the total content of the resin component and the dispersant. When the amount is less than 0.5% by mass, the dispersion of the resin component becomes insufficient, and the dispersion stability may decrease. When the amount exceeds 20% by mass, the physical properties of the obtained coating film may decrease. More preferably, it is 1.0 to 10% by mass.
[0048]
The first aqueous base paint contains an oxazoline group-containing compound as a curing agent. The oxazoline group-containing compound is not particularly limited, and examples thereof include commercially available compounds.
[0049]
Further, the oxazoline group-containing compound used in the present invention is preferably a compound having two or more 2-oxazoline groups, for example, a polycarboxylic acid such as dicarboxylic acid, a nitrile compound such as dinitrile and trinitrile, and ethanol. Examples thereof include an oxazoline compound (a) obtained by a reaction with an amine and an oxazoline group-containing polymer (b). One or more of these can be used.
[0050]
The 2-oxazoline group is represented by the formula (1).
[0051]
Embedded image

[0052]
(In the above formula, R1, R2, R3 and R4 are each independently a hydrogen atom, halogen, alkyl, aralkyl, phenyl or substituted phenyl group.) That is, the 2-oxazoline group is one or more of hydrogen May be substituted.
[0053]
Examples of the oxazoline compound (a) include 2,2′-bis- (2-oxazoline), 2,2′-methylene-bis- (2-oxazoline), and 2,2′-ethylene-bis- (2 -Oxazoline), 2,2'-trimethylene-bis- (2-oxazoline), 2,2'-tetramethylene-bis- (2-oxazoline), 2,2'-hexamethylene-bis- (2-oxazoline) 2,2'-octamethylene-bis- (2-oxazoline), 2,2'-ethylene-bis- (4,4'-dimethyl-2-oxazoline), 2,2'-p-phenylene-bis- (2-oxazoline), 2,2′-m-phenylene-bis- (2-oxazoline), 2,2′-m-phenylene-bis- (4,4′-dimethyl-2-oxazoline), bis- ( 2-oxazo Sulfonyl cyclohexane) sulfide, bis - (2-like oxazolinyl sulfonyl norbornane) sulfide and the like, or using any alone, can be used in combination of two or more kinds.
[0054]
The oxazoline group-containing polymer (b) is obtained by polymerizing addition polymerizable oxazoline and, if necessary, at least one other polymerizable monomer. The addition-polymerizable oxazoline is represented by the formula (2).
[0055]
Embedded image

[0056]
(In the above formula, R1, R2, R3 and R4 are each independently a hydrogen atom, halogen, alkyl, aralkyl, phenyl or substituted phenyl group, and R5 is a non-cyclic organic group having an addition-polymerizable unsaturated bond. Is.)
[0057]
Examples of the addition polymerizable oxazoline include, for example, 2-vinyl-2-oxazoline, 2-vinyl-4-methyl-2-oxazoline, 2-vinyl-5-methyl-2-oxazoline, 2-isopropenyl-2 -Oxazoline, 2-isopropenyl-4-methyl-2-oxazoline, 2-isopropenyl-5-ethyl-2-oxazoline and the like. One compound selected from these groups may be used alone. Or a mixture of two or more compounds. Among them, 2-isopropenyl-2-oxazoline is preferred because it is easily available industrially. The amount of the addition-polymerizable oxazoline used when the oxazoline group-containing polymer (b) is used as the oxazoline group-containing compound is not particularly limited. It is preferable that the content is at least mass%. If the amount is less than 1% by mass, the degree of curing is insufficient, and the durability and water resistance tend to be impaired.
[0058]
The other monomer used in the present invention is not particularly limited as long as it is a monomer copolymerizable with the addition-polymerizable oxazoline and does not react with the oxazoline group. For example, methyl (meth) acrylate (Meth) acrylates such as butyl (meth) acrylate and 2-ethylhexyl (meth) acrylate; unsaturated nitriles such as (meth) acrylonitrile; (meth) acrylamide, N-methylol (meth) acrylamide and the like Unsaturated amides; vinyl esters such as vinyl acetate and vinyl propionate; vinyl ethers such as methyl vinyl ether and ethyl vinyl ether; α-olefins such as ethylene and propylene; halogens such as vinyl chloride, vinylidene chloride and vinyl fluoride. Α, β-unsaturated monomers; α, β- such as styrene and α-methylstyrene Etc. saturated aromatic monomers such may be mentioned, it is possible to use these one compound or a mixture of two or more thereof.
[0059]
The oxazoline group-containing polymer (b) can be produced from an addition-polymerizable oxazoline and, if necessary, at least one other monomer by a conventionally known polymerization method such as suspension polymerization, solution polymerization, and emulsion polymerization. The supply form of the oxazoline group-containing compound may be any form such as an organic solvent solution, an aqueous solution, a non-aqueous dispersion, or an emulsion.
[0060]
For example, the water-soluble polymer can be produced by solution polymerization of an addition-polymerizable oxazoline and, if necessary, at least one other monomer in an aqueous medium by a conventionally known polymerization method. The aqueous medium that can be used is not particularly limited as long as it can be mixed with water. For example, water; or water and methanol, ethanol, propanol, isopropanol, butanol, tert-butanol, ethylene glycol, ethylene glycol A mixed solution with monomethyl ether, ethylene glycol monobutyl ether, diethylene glycol, acetone, methyl ethyl ketone and the like can be mentioned.
[0061]
In order to impart water solubility to the polymer, the proportion of the hydrophilic monomer in the monomer mixture is 50% by mass or more, preferably 70% by mass or more. The hydrophilic monomers are 2-hydroxyethyl (meth) acrylate, methoxypolyethylene glycol (meth) acrylate, and 2-aminoethyl (meth) acrylate among addition-polymerizable oxazolines and other monomers. And salts thereof, (meth) acrylamide, N-methylol (meth) acrylamide, N- (2-hydroxyethyl) (meth) acrylamide, (meth) acrylonitrile, sodium styrenesulfonate and the like.
[0062]
In the first aqueous base coating composition of the present invention, the solid content mass ratio of the acrylic emulsion resin to the oxazoline group-containing compound is preferably 99/1 at an upper limit and 70/30 at a lower limit, and is 98/2 at an upper limit and 80 / at a lower limit. More preferably, it is 20. If the above ratio exceeds 99/1, the curability may decrease, and if it is less than 70/30, various properties of the obtained coating film may decrease.
[0063]
As the component that can be contained as a film-forming resin in addition to the acrylic emulsion resin and the oxazoline group-containing compound, as described above, a volatile basic substance, the unit area at the time of forming a multilayer coating film 1mm²7x10 volatile basic substances per unit^-6There is no particular limitation as long as it can be formed so as to be not more than mmol, and for example, a solvent type thermosetting resin or the like can be contained.
[0064]
The solvent-type thermosetting resin is not particularly limited, and examples thereof include an acrylic resin; a polyester resin; an alkyd resin; a modified alkyd resin such as an epoxy-modified resin, a rosin-modified resin, and a phenol resin-modified resin; a polyurethane resin; a polyolefin resin; Resins used in ordinary solvent-type paints can be used, and one or more of these can be used. Among them, acrylic resin, polyester resin and polyurethane resin can be preferably used from the viewpoint of pigment dispersibility, water resistance and weather resistance.
[0065]
The first aqueous base coating composition does not include a pigment and a volatile basic substance in the solid content of the pigment dispersant, or the content of the volatile basic substance is 3% by mass or less. And a dispersant pigment dispersion paste obtained by previously dispersing
[0066]
In the present invention, by using the pigment dispersant, the amount of the volatile basic substance in the first base coating film formed from the first aqueous base coating material is reduced, and yellowing of the obtained multilayer coating film is reduced. Can be suppressed. That is, when the pigment dispersant used contains a volatile basic substance in an amount exceeding 3% by mass based on the solid content of the pigment dispersant, the obtained multilayer coating film turns yellow and the finished appearance is poor.
[0067]
The volatile basic substance means a basic substance having a boiling point of 300 ° C. or lower, and includes inorganic and organic nitrogen-containing basic substances. Specifically, the first aqueous base paint In the description. When the above basic substance is contained, a unit area of 1 mm²The amount of volatile basic substance per unit is 7 × 10^-6It is necessary to use a pigment dispersant such that the amount becomes less than mmol. As described in the description of the acrylic emulsion resin, the components other than the pigment dispersant may also contain a volatile basic substance, and thus the amount of the volatile basic substance contained in the pigment dispersant Is preferably reduced.
[0068]
That is, it is preferable to disperse the volatile basic substance using a pigment dispersant substantially free of a pigment dispersant. preferable.
[0069]
The pigment dispersant is a resin having a structure including a pigment affinity portion and a hydrophilic portion. Examples of the pigment affinity portion and the hydrophilic portion include, for example, nonionic, cationic and anionic functional groups, and one molecule of the pigment dispersant may have two or more types of the functional groups. .
[0070]
Examples of the nonionic functional group include a hydroxyl group, an amide group, and a polyoxyalkylene group. Examples of the cationic functional group include an amino group, an imino group, and a hydrazino group. In addition, examples of the anionic functional group include a carboxyl group, a sulfonic acid group, and a phosphate group. Such a pigment dispersant can be manufactured by a method well known to those skilled in the art.
[0071]
The pigment dispersant is not particularly limited as long as it does not contain a volatile basic substance in the solid content of the pigment dispersant or is 3% by mass or less. What can disperse a pigment is preferable.
[0072]
For example, commercially available products (all of which are trade names below) can be used. Specifically, Disperbyk @ 190, Disperbyk @ 181, and Disperbyk @ 182 which are anionic / nonionic dispersants manufactured by Big Chemie Co., Ltd. Polymer), Disperbyk # 184 (polymer copolymer), EFKAPOLYMER 4550, an anionic / nonionic dispersant manufactured by EFKA, Solsperse 27,000, a nonionic dispersant manufactured by Avicia, and Solsperse 41000, an anionic dispersant, Solsperse 53095 and the like.
[0073]
It is preferable that the number average molecular weight of the pigment dispersant has a lower limit of 1,000 and an upper limit of 100,000. If it is less than 1000, the dispersion stability may not be sufficient, and if it exceeds 100,000, the viscosity may be too high and handling may be difficult. More preferably, the lower limit is 2,000 and the upper limit is 50,000, and further preferably, the lower limit is 4,000 and the upper limit is 50,000.
[0074]
The pigment dispersant is mixed and dispersed together with the pigment according to a known method to obtain a dispersant pigment dispersion paste. The proportion of the pigment dispersant in the composition at the time of production of the dispersant pigment dispersion paste is preferably 1% by mass (lower limit) and 20% by mass (upper limit) based on the solid content of the dispersant pigment dispersion paste. If it is less than 1% by mass, the pigment cannot be dispersed stably, and if it exceeds 20% by mass, the physical properties of the coating film may be poor. Preferably, the lower limit is 5% by mass and the upper limit is 15% by mass.
[0075]
The pigment is not particularly limited as long as it is a pigment used in ordinary aqueous base paints, but is preferably a colored pigment from the viewpoint of improving weather resistance and securing hiding properties. In particular, titanium dioxide is more preferable because it is excellent in color concealing property and inexpensive.
[0076]
Examples of the pigments other than titanium dioxide include, for example, azochelate pigments, insoluble azo pigments, condensed azo pigments, phthalocyanine pigments, indigo pigments, perinone pigments, perylene pigments, dioxane pigments, quinacridone pigments, Organic color pigments such as linone pigments, diketopyrrolopyrrole pigments, benzimidazolone pigments, and metal complex pigments; and inorganic color pigments such as graphite, yellow iron oxide, red iron oxide, carbon black, and the like. As the above-mentioned pigment, extenders such as calcium carbonate, barium sulfate, clay and talc may be used.
[0077]
It can also be used as a standard gray paint having carbon black and titanium dioxide as main pigments as the above-mentioned pigment, and can also be used a paint that combines lightness or hue with a top coat paint or a paint that combines various color pigments. .
[0078]
In the first aqueous base paint, the pigment preferably has a weight ratio (PWC) of the pigment to the total weight of the pigment and the resin solid content (PWC) of 10 to 60% by mass. If the amount is less than 10% by mass, the concealing property may be reduced due to insufficient pigment. If the amount exceeds 60% by mass, the viscosity at the time of curing is increased due to excessive pigment, and the flowability is reduced, and the appearance of the coating film may be reduced.
[0079]
The content of the pigment dispersant in the first aqueous base paint is preferably 0.5% by mass as the lower limit and 10% by mass as the upper limit based on the solid content. When the amount is less than 0.5% by mass, the dispersion amount of the pigment may be poor due to a small amount of the pigment dispersant. If it exceeds 10% by mass, the properties of the coating film may be poor. Preferably, the lower limit is 1% by mass and the upper limit is 5% by mass.
[0080]
The first aqueous base paint contains a thickener. Thereby, the viscosity of the first aqueous base paint can be increased, and when the first aqueous base paint is applied in the step (1), it is possible to suppress the occurrence of sagging. In addition, a mixed layer between the first base coating film and the second base coating film can be further suppressed. As a result, compared with the case where no thickener is contained, the coating workability at the time of coating is improved, and the finished appearance of the obtained coating film can be made excellent.
[0081]
The thickener is not particularly limited. For example, viscose, methylcellulose, ethylcellulose, and hydroxyethylcellulose as cellulosic substances, and Tyrose MH and Tyrose H (all commercially available from Hoechst Co., Ltd., Sodium polyacrylate, polyvinyl alcohol, carboxymethylcellulose as the alkali-thickening type, and Primal ASE-60, Primal TT-615, Primal RM as commercially available products (hereinafter, all trade names). -5 (all manufactured by Rohm & Haas Co., Ltd.), Eucher Polyforb (manufactured by Union Carbide Co., Ltd.) and the like; polyvinyl alcohol, polyethylene oxide, and commercially available products (hereinafter, all trade names) , Adecanol U -420, Adekanol UH-462, Adekanol UH-472, UH-540, Adekanol UH-814N (manufactured by Asahi Denka Kogyo), Primal RH-1020 (manufactured by Rohm & Haas), Kuraray Povar (manufactured by Kuraray), and the like. Can be mentioned. Among them, an association-type thickener is preferable. These may be used alone or in combination of two or more.
[0082]
It is particularly preferred that the thickener is a urethane-associated thickener. In the case where the urethane-associated thickener is contained in the first aqueous base coating composition of the present invention, the hydrophobic portions in the urethane-associated thickener are associated with each other in a water-soluble solvent, whereby the effect is improved. It is presumed to exhibit a thickening effect. Thereby, compared with the case of using a cellulosic thickener, the paint is more effectively thickened, as a result, it is possible to suppress sagging at the time of painting, and also to improve the thixotropic properties of the paint Thus, the appearance of the obtained coating film can be made excellent.
[0083]
In the present invention, the urethane-associated thickener means an amphiphilic thickener containing both a hydrophilic portion and a hydrophobic portion, and includes, for example, one or more mono- or polyisocyanates. And one or more polyether monoalcohols as a raw material.
[0084]
The mono- or polyisocyanate is not particularly limited as long as it has one or more isocyanate groups in one molecule, and may be used alone or in combination of two or more.
[0085]
The polyether monoalcohol is not particularly limited as long as it is a polyether of a branched or secondary monohydric alcohol. Such a compound can be obtained by addition polymerization of a branched or secondary monohydric alcohol with an alkylene oxide or styrene oxide. The term “branched chain or secondary monohydric alcohol” as used herein is not particularly limited, but, for example, the total number of carbon atoms is preferably a lower limit of 8 and an upper limit of 36, more preferably a lower limit of 12 and an upper limit of 24.
[0086]
The alkylene oxide or styrene oxide to be added may be one obtained by homopolymerization, two or more kinds of random polymerization or block polymerization. The polymerization degree preferably has a lower limit of 1 and an upper limit of 500, more preferably a lower limit of 10 and an upper limit of 200, from the viewpoint of the appearance of the obtained coating film.
[0087]
As a method for obtaining the urethane associative thickener, for example, a mono- or polyisocyanate and a polyether monoalcohol are prepared by adjusting the ratio of the hydroxyl value from each compound to the isocyanate value to 1.05 / 1 to 1.4 /. 1, for example, a method of heating and reacting at 80 to 90 ° C. for 1 to 3 hours in the same manner as in the usual reaction between polyether and isocyanate.
[0088]
As a commercially available product of the urethane associative thickener, for example, from the viewpoint of preventing sagging at the time of coating, of the ADEKANOL series, UH-814N, UH-462, UH-420, UH-472, UH-540. (Manufactured by Asahi Denka Kogyo Co., Ltd., all of which are trade names).
[0089]
The content of the thickener in the first aqueous base paint is preferably a lower limit of 0.01 parts by weight and an upper limit of 20 parts by weight based on 100 parts by weight of the resin solid content of the first aqueous base paint. More preferably, the amount is 0.1 part by mass, and the upper limit is 10 parts by mass. When the amount is less than 0.01 part by mass, the thickening effect is not obtained, and sagging at the time of coating may occur. When the amount is more than 20 parts by mass, the appearance and various properties of the obtained coating film may be reduced. is there.
[0090]
The first aqueous base paint may further contain additive components such as an ultraviolet absorber, an antioxidant, an antifoaming agent, a surface conditioner, and an anti-bake agent.
[0091]
First aqueous base coating film forming method
The first aqueous base paint is used for forming a first base coating film on an object to be coated. The object to be coated (material) is not particularly limited, and may be, for example, a steel plate member and a plastic member used in an automobile.
[0092]
Examples of the steel plate member include metals such as iron, aluminum, copper, tin, zinc, and stainless steel, alloys and castings containing them, and examples of the plastic member include, for example, molding of polyurethane, polypropylene, and polycarbonate. Bodies and foams.
[0093]
Specifically, the parts where the steel plate member is used include doors, roofs, hoods, fenders, trunks, and the like, and the parts where the plastic member is used include bumpers, fascias, mirrors, moldings, guards, and the like. No. The term “car” includes not only passenger cars but also trucks, motorcycles, buses, bicycles, and the like.
[0094]
Further, an electrodeposition coating film can be formed as a primer coating film on a steel plate member. In that case, a cationic or anionic electrodeposition coating may be used. The cationic type is preferable because a coating film having excellent corrosion resistance can be obtained. Further, it is particularly preferable that the steel sheet member is subjected to a chemical conversion treatment with a phosphate, a chromate or the like before performing the electrodeposition coating from the viewpoint of rust prevention.
[0095]
The method of applying the first aqueous base paint is not particularly limited, and includes, for example, air spray; air electrostatic spray commonly called “react gun”; commonly called “micro-micro (μμ) bell”, “micro (μ) bell”. , "Metabell" and the like, and a rotary atomizing electrostatic coating machine.
[0096]
The dry film thickness of the first aqueous base coating film varies depending on the application, but is preferably 5 to 50 μm. If it exceeds the upper limit, sharpness may be reduced, or sagging during coating or troubles such as abrasion may occur at the time of heat curing. If it is below the lower limit, the appearance may be deteriorated. For the purpose of obtaining a good finished appearance, it is preferable to perform preheating after applying the first aqueous base paint.
[0097]
Step (2)
The step (2) is a step of applying a second aqueous base paint on the first base coat formed as described above to form a second base coat.
[0098]
Second aqueous base paint
The second aqueous base paint is mainly applied to impart and maintain aesthetic and design properties such as color and glitter to the multilayer coating film.
[0099]
In the present invention, the pigment and the pigment dispersant containing no or no more than 3% by mass of a volatile basic substance in the solid content of the pigment dispersant are also dispersed in advance as the second aqueous base paint. It is preferable to use one containing a dispersant / pigment dispersed paste obtained as described above. That is, a unit area of 1 mm before heating and curing when a multilayer coating film was formed.²Around 7 × 10^-6There is no particular limitation as long as it can be formed so as to be not more than mmol. Examples of the volatile basic substance and the pigment dispersant include those exemplified in the description of the first aqueous base paint.
[0100]
As the pigment contained in the second aqueous base paint, the coloring pigments and extenders described in the description of the first aqueous base paint can be used. For example, a color pigment and / or extender such as red, blue or black may be blended and used as a solid base paint, and a glitter pigment may be further blended and used as a metallic base paint.
[0101]
The brilliant pigment is not particularly limited. For example, an uncolored or colored metallic brilliant such as a metal or an alloy and a mixture thereof, interference mica powder, colored mica powder, white mica powder, graphite or a colorless colored flat pigment And the like. The glitter pigment is excellent in dispersibility and can form a coating film excellent in transparency and glitter. Specific examples of the metal include aluminum, aluminum oxide, copper, zinc, iron, nickel, and tin.
[0102]
The shape of the glittering pigment is not particularly limited, and may be colored.₅₀) Has a lower limit of 2 μm and an upper limit of 50 μm, and the thickness is preferably a scale-like one having a lower limit of 0.1 μm and an upper limit of 5 μm. The average particle size is more preferably a lower limit of 10 μm and an upper limit of 35 μm because of excellent glitter.
[0103]
One or more of the above pigments can be used, and one or more of a combination of a coloring pigment and an extender pigment, and, if necessary, a flat pigment and a glitter pigment are used. Can be.
[0104]
In the second aqueous base paint, the proportion of the pigment dispersant in the composition at the time of manufacturing the dispersant pigment dispersion paste is preferably 3 to 50% by mass based on the solid content of the dispersant pigment dispersion paste. If it is less than 3% by mass, the pigment cannot be dispersed stably, and if it exceeds 50% by mass, the physical properties of the obtained coating film may be deteriorated.
[0105]
The second aqueous base paint can be prepared by mixing the dispersant-pigment-dispersed paste, a film-forming resin, and a curing agent.
[0106]
The film-forming resin, curing agent, and other additives used in the second aqueous base paint are not particularly limited, and may include those described in the first aqueous base paint. Specifically, a combination of an acrylic resin and / or a polyester resin and a melamine resin is preferred from the viewpoint of pigment dispersibility and workability. In addition, the method for preparing the second aqueous base paint can also include the method exemplified in the first aqueous base paint.
[0107]
The pigment concentration (PWC) in the second aqueous base paint including the glitter pigment and all other pigments generally has a lower limit of 0.1% by mass and an upper limit of 50% by mass, and preferably a lower limit of 50% by mass. The upper limit is 0.5% by mass and the upper limit is 40% by mass, and more preferably the lower limit is 1% by mass and the upper limit is 30% by mass. If it exceeds 50% by mass, the appearance of the coating film is reduced.
[0108]
The content of the pigment dispersant in the second aqueous base paint is such that the solid content of the pigment dispersant does not contain a volatile basic substance as a pigment dispersant or contains 3% by mass or less. When used, the lower limit is preferably 1% by mass and the upper limit is 20% by mass on a solid content basis. If the amount is less than 1% by mass, the dispersion amount of the pigment may be poor due to a small amount of the pigment dispersant. If it exceeds 20% by mass, the physical properties of the obtained coating film may be reduced.
[0109]
Second base coating film forming method
The second aqueous base coating is used to form a second base coating by coating on the first base coating formed by the above method.
[0110]
As the above-mentioned coating method, the method exemplified when applying the first aqueous base paint can be exemplified. When the second base paint is applied to an automobile body or the like, in order to enhance the design, multi-stage painting by air electrostatic spraying, preferably two-stage painting or air electrostatic spraying is used. It is preferable to carry out by a coating method in which the above-mentioned and a rotary atomizing type electrostatic coating machine are combined.
[0111]
The dry film thickness of the second base coating film varies depending on the application, but is preferably 5 to 35 μm. If it exceeds the upper limit, the sharpness may deteriorate, or problems such as unevenness and flow may occur at the time of coating. If it is less than the lower limit, the film may be cut or the color may be uneven.
[0112]
Step (3)
The step (3) is a step of further applying a clear paint on the second base coat formed as described above to form a clear coat.
[0113]
Clear paint
The clear coating, when using a metallic base paint containing a brilliant pigment as the second base paint, smoothes the unevenness of the base coating due to the brilliant pigment, shine, etc. It is formed to protect and improve the appearance.
[0114]
The clear coating is not particularly limited, and includes, for example, a coating composed of a film-forming resin, a curing agent, and other additives.
[0115]
The film-forming resin is not particularly limited, and examples thereof include an acrylic resin, a polyester resin, an epoxy resin, and a urethane resin. These may be used in combination with a curing agent such as an amino resin and / or a polyisocyanate resin. Can be. From the viewpoint of transparency or acid etching resistance, etc., a combination of an acrylic resin and / or a polyester resin with an amino resin and / or a polyisocyanate resin, or an acrylic resin and / or a polyester resin having a carboxylic acid / epoxy group curing system It is preferable to use
[0116]
As the clear paint, after applying the second base paint described above, in order to apply in the state of a wet paint film, to prevent the familiarity and inversion of the paint film layer, or to prevent sagging, a viscosity control agent as an additive It is preferred to contain. The addition amount of the viscosity control agent is a lower limit of 0.01 part by mass and an upper limit of 10 parts by mass, preferably 0.02 parts by mass of lower limit and 8 parts by mass of upper limit, based on 100 parts by mass of the resin solid content of the clear paint. More preferably, the lower limit is 0.03 parts by mass and the upper limit is 6 parts by mass. If the amount exceeds 10 parts by mass, the appearance of the obtained coating film is deteriorated. If the amount is less than 0.1 part by mass, the effect of controlling the viscosity cannot be obtained, which causes problems such as sagging at the time of forming the coating film.
[0117]
The coating form of the clear coating may be any of an organic solvent type, an aqueous type (water-soluble, water-dispersible, emulsion), a non-aqueous dispersion type, and a powder type. If necessary, a curing catalyst, a surface conditioner, etc. Can be used.
[0118]
Clear coating film forming method
The method for preparing and coating the clear paint can be performed according to a conventional method. The dry film thickness of the clear coating film varies depending on the application, but is 10 to 70 μm. When the dry film thickness exceeds the upper limit, sharpness may be reduced, or unevenness or flow may occur at the time of coating. When the dry film thickness is lower than the lower limit, the appearance of the obtained coating film may be deteriorated. .
[0119]
Step (4)
In the step (4), the first base coating, the second base coating, and the clear coating are simultaneously heated and cured to obtain a multilayer coating.
[0120]
By performing the heating and curing at a lower limit of 80 ° C and an upper limit of 160 ° C, preferably at a lower limit of 90 ° C and an upper limit of 140 ° C, a cured coating film having a high degree of crosslinking can be obtained. If it exceeds 160 ° C., the coating film becomes hard and brittle, and if it is less than 80 ° C., curing is not sufficient. The curing time varies depending on the curing temperature, but is suitably from 90 to 140 ° C. for 10 to 60 minutes.
[0121]
In the method for forming a multilayer coating film of the present invention, the multilayer coating film formed by the above steps (1), (2) and (3) has a unit area of 1 mm before performing the step (4).²The total amount of volatile basic substances is 7 × 10^-6mmol or less.
[0122]
In the multilayer coating film obtained by the coating in the above steps (1), (2) and (3), volatile basic substances contained in the first aqueous base paint and the second aqueous base paint remain. I do. The total amount of the remaining volatile basic substances is 1 mm in unit area of the coating film.²At 7 × 10^-6When the amount exceeds mmol, heating to cure the multilayer coating film causes a chemical change and yellowing due to the remaining volatile basic substance. In addition, since the basic substance volatilized from the first base coating film or the second base coating film is trapped inside the clear coating film, and the above-mentioned clear coating film also turns yellow, the color reproducibility and the design are reduced. I do. Preferably, 6.5 × 10^-6mmol or less.
[0123]
In this specification, the unit area is 1 mm²In the above, the total amount (mmol) of the volatile basic substance refers to 1 mm on the surface of the multilayer coating film formed by the steps (1), (2) and (3).²Assuming a section (S ′) obtained by projecting a section (S) having an area of perpendicularly onto the coating surface of the object to be coated, the section (S) is sandwiched between the section (S) and the section (S ′). Means the content (mmol) of the total volatile basic substance contained in the portion (V).
[0124]
1mm above unit area²The total amount (mmol) of the above-mentioned volatile basic substance in the above-mentioned (V) is the amount of the volatile basic substance contained in each coating film of the first base coating film, the second base coating film and the clear coating film. Amount (mmol). That is, the amount [Ammol] of the volatile basic substance contained in the first base coating film in the portion (V) and the volatile basic content contained in the second base coating film in the portion (V). The total amount [(A + B + C) mmol] of the amount [Bmmol] of the substance and the amount [Cmmol] of the volatile basic substance contained in the clear coating film in the portion (V) is referred to as the coating film in the present specification. Unit area 1mm²Is the total amount of the volatile basic substances.
[0125]
1mm above unit area²In the above, the total amount (mmol) of the volatile basic substance is determined by collecting a sample of the multilayer coating film formed in the above steps (1), (2) and (3), and performing gas chromatography on the collected sample. From the content of the volatile basic substance determined in the above and the dry film thickness, 1 g was 1 cm³Can be obtained by assuming that
[0126]
The film thickness of the multi-layer coating film obtained by the coating film forming method of the present invention is usually lower limit 30 μm, upper limit 300 μm, preferably lower limit 50 μm, upper limit 250 μm. If it exceeds 300 μm, the physical properties of the film such as a thermal cycle decrease, and if it is less than 30 μm, the strength of the film itself decreases.
[0127]
The first aqueous base paint of the present invention is a paint used for forming a first base coat in the method for forming a multilayer paint film, wherein the first aqueous base paint is an acrylic emulsion resin, an oxazoline group-containing compound. , A dispersant, a pigment-dispersed paste and a thickener. In the method for forming a multilayer coating film, by using the first aqueous base coating material, excellent water resistance, solvent resistance, coating performance such as chipping resistance, and sagging during coating and yellowing of the coating film are prevented. And a coating film having an excellent appearance can be obtained.
[0128]
Further, in the method for forming a multilayer coating film of the present invention, the first aqueous base paint contains an acrylic emulsion resin, an oxazoline group-containing compound, a dispersant pigment-dispersed paste, and a thickener. ), The coating film formed by the steps (2) and (3) has a unit area of 1 mm before performing the step (4).²The total amount of volatile basic substances is 7 × 10^-6Since the amount is not more than mmol, sagging during coating and yellowing of the coating film can be suppressed, and a multilayer coating film having an excellent appearance can be obtained. In particular, when a urethane associative thickener is used as a thickener, a coating film having a more excellent finished appearance can be obtained because sagging during coating can be more effectively suppressed.
[0129]
Further, the three-wet coating method of the present invention can eliminate the step of heating and curing the base coating film as compared with the conventional three-coat two-bake method, thereby shortening the process, reducing costs, and reducing energy consumption. And a new coating system aiming at reduction of environmental load can be constructed.
[0130]
【Example】
Hereinafter, the present invention will be described more specifically with reference to examples, but the present invention is not limited to these examples. In the examples, “parts” means “parts by mass” unless otherwise specified.
[0131]
Production Example 1 Production of color pigment paste
9.4 parts of a commercially available dispersant "Disperbyk @ 190" (a nonionic / anionic dispersant manufactured by BYK Chemie, trade name), 36.8 parts of ion-exchanged water, 34.5 parts of rutile-type titanium dioxide, 34.4 parts of barium sulfate, and After premixing 6 parts of talc, a glass bead medium was added in a paint conditioner, and mixed and dispersed at room temperature until the particle size became 5 μm or less, to obtain a dispersant / color pigment dispersed paste.
[0132]
Production Example 2 Production of acrylic emulsion resin
35.75 parts of deionized water was added to the reaction vessel, and the temperature was raised to 80 ° C. while mixing and stirring in a nitrogen stream. Then, as an α, β-ethylenically unsaturated monomer mixture, 10 parts of styrene, 24.02 parts of methyl methacrylate, 28.94 parts of butyl acrylate, 20.11 parts of ethyl acrylate, and 15.40 parts of 4-hydroxybutyl acrylate Parts, 1.53 parts of methacrylic acid, 3.0 parts of AQUALON HS-10 (polyoxyethylene alkylpropenyl phenyl ether sulfate, an anionic dispersant manufactured by Daiichi Kogyo Seiyaku Co., Ltd., trade name), Adecaria Soap NE-20 ( Nonionic dispersant manufactured by Asahi Denka Kogyo Co., Ltd., a monomer emulsion composed of 0.5 part of deionized water and 50 parts of deionized water, an initiator solution composed of 0.30 part of ammonium peroxy acid and 15.0 parts of deionized water Was dropped into the reaction vessel in parallel over 2 hours. After completion of dropping, aging was performed at the same temperature for 2 hours. Next, the mixture was cooled to 40 ° C. and filtered through a 400 mesh filter to obtain an acrylic emulsion resin having a particle size of 200 nm, a nonvolatile content of 50%, an acid value of 10 mg KOH / g, and a hydroxyl value of 60.
[0133]
Example 1
(1) Preparation of first aqueous base paint 1
As shown in Table 1, 60.3 parts of the dispersant / color pigment dispersion paste obtained in Production Example 1, 111.4 parts of the acrylic emulsion resin obtained in Production Example 2, Epocross WS-500 (Nippon Shokubai Co., Ltd.) After mixing 5.5 parts of an oxazoline group-containing compound, a water-soluble acrylic copolymer, and an oxazoline value of 220 [solid / eq] (trade name), ADECANOL UH-814N (urethane-associated thickener, active ingredient 30) %, Manufactured by Asahi Denka Kogyo Co., Ltd. (trade name) (1.0 part) was mixed and stirred to obtain a first aqueous base paint 1.
[0134]
[Table 1]

[0135]
(2) Coating method
A power top U-50 (cationic electrodeposition paint, trade name, manufactured by Nippon Paint Co., Ltd.) is electrodeposited on a zinc phosphate-treated dull steel sheet so that the dry coating film becomes 20 μm, and heated at 160 ° C. for 30 minutes. After hardening, it was cooled to obtain a steel plate substrate. In addition, a 10 x 20 cm test piece made of a polypropylene material for automobile bumpers that had been degreased in advance was prepared as a plastic member.
[0136]
The first aqueous base paint 1 was applied to both substrates by air spray coating at a thickness of 20 μm and preheated at 80 ° C. for 5 minutes. (Trade name) was applied by air spray coating at 10 μm and preheated at 80 ° C. for 3 minutes. Further, as a clear paint, R298 clear (isocyanate-curable clear paint, trade name, manufactured by Nippon Bee Chemical Co., Ltd., trade name) was applied on the coated plate by 35 μm by air spray coating, followed by heat curing at 100 ° C. for 30 minutes to prepare a test piece. Obtained.
[0137]
In addition, the amount of the basic substance remaining in the multilayer coating film before curing was measured by a method described later, and was 4.4 mmol. Furthermore, the multilayer coating film obtained after the heat curing was excellent in the finished appearance, and was satisfactory without any change from the viewpoint of yellowing of the coating film.
[0138]
The first aqueous base paint (first aqueous base paint 1), the second aqueous base paint (Aquarex AR-2000 silver metallic), and the clear paint (R298 clear) were diluted under the following conditions and used for painting.
[0139]
(1st water base paint)
Thinner: ion exchanged water
40 seconds / NO. 4 Ford cup / 20 ℃
(2nd water base paint)
Thinner: ion exchanged water
45 seconds / NO. 4 Ford cup / 20 ℃
(Clear paint)
Thinner: EEP (ethoxyethyl propionate) / S-150 (exxon aromatic solvent, trade name) = 1/1
30 seconds / NO. 4 Ford cup / 20 ℃
[0140]
(3) Evaluation of coating film
(Water resistance test)
The obtained coated steel sheet was immersed in warm water of 40 ° C. for 10 days, and the appearance after 1 hour of washing was visually observed and evaluated according to the following criteria.
Appearance evaluation criteria
5; no change
4: The warm water boundary swells slightly
3: Warm water boundary is slightly darkened
2: Warm water boundary is darkened
1: Warm water boundary swells and the coating is dark
[0141]
[Chip resistance]
Evaluation of the chipping resistance of the obtained coated steel sheet was performed as follows. Using a gravure tester tester (manufactured by Suga Test Instruments Co., Ltd.), 300 crushed stones of No. 7 were 3.0 kgf / cm from a distance of 35 cm.²At an angle of 45 °. After washing and drying, a peeling test was performed using an industrial gum tape manufactured by Nichiban Co., Ltd., and thereafter, the degree of peeling of the coating film was visually observed and evaluated.
Evaluation criteria
5; no peeling
4: Small peeling area and low frequency
3: The peeling area is small, but the frequency is somewhat high
2: Large peeling area but low frequency
1: Large peeling area and frequent
[0142]
(Basic substance amount measurement method)
Samples of the first base coating film and the second base coating film were sampled after the first aqueous base paint and the second aqueous base paint were applied and preheated in the above coating method, respectively. After weighing 0.3 g of the collected sample, adding 0.6 ml of a standard solution (1.0 g of isobutanol to 100 ml of methanol), sufficiently agitating with a touch mixer and ultrasonic vibration, further centrifuging, and then supernatant. The liquid was collected, and the amount of the basic substance was quantified by gas chromatography. From the amount of the obtained basic substance and the dry film thickness, 1 g was 1 cm³And the unit area of the coating film is 1 mm²The total amount of the basic substances in was calculated.
[0143]
(Adhesion evaluation)
Using a cutter knife, 100 cuts were formed on the obtained polypropylene coated plate so that each piece reached a base material in a grid of 2 mm, and a cellophane tape manufactured by Nichiban Co., Ltd. was pressed on the surface with a thumb. Thereafter, the film was vigorously peeled off in the upward 45 ° direction, and the adhesion of the coating film to each member was tested. The results were determined according to the following criteria.
Adhesion evaluation criteria
5; not peel at all
4: 1 to less than 10 peelings
3: More than 10 pieces and less than 50 pieces
2: More than 50 pieces and less than 90 pieces
1: 90 or more peeling
Table 2 shows the evaluation results.
[0144]
Examples 2 to 4
A test piece was obtained in the same manner as in Example 1 except that a multilayer coating film was prepared by a combination of paints and a curing method shown in Table 2 below. In Example 4, MacFlow # O-1800W-2 Clear (acid epoxy-curable clear paint, trade name, manufactured by Nippon Paint Co., Ltd.) was used as the clear paint. In addition, evaluation was performed in the same manner, and the results are shown in Table 2.
[0145]
Example 5
Example 1 Example 1 was repeated except that a first aqueous base paint was prepared by the method shown below, and MacFlow # O-1800W-2 Clear (acid epoxy-curable clear paint, trade name, manufactured by Nippon Paint Co., Ltd.) was used as the clear paint. A test piece was obtained in the same manner as described above. In addition, evaluation was performed in the same manner, and the results are shown in Table 2.
[0146]
(1) Preparation of first aqueous base paint 2
As shown in Table 1, 60.3 parts of the dispersant / color pigment dispersion paste obtained in Production Example 1, 111.4 parts of the acrylic emulsion resin obtained in Production Example 2, and 2,2 as the oxazoline group-containing compound. After mixing 1.0 part of '-m-phenylene-bis- (2-oxazoline), adecanol UH-814N (urethane-associated thickener, active ingredient 30%, trade name, manufactured by Asahi Denka Kogyo KK) 0 parts were mixed and stirred to obtain a first aqueous base paint 2.
[0147]
Comparative Example 1
A test piece was obtained in the same manner as in Example 1 except that the first aqueous base paint was prepared by the method described below. In addition, evaluation was performed in the same manner, and the results are shown in Table 2.
[0148]
(1) Preparation of first aqueous base paint 3
As shown in Table 1, 60.3 parts of the dispersant coloring pigment dispersion paste obtained in Production Example 1, 81.1 parts of the acrylic emulsion resin obtained in Production Example 2, Cymel 327 (Imino manufactured by Mitsui Cytec Co., Ltd.) 19.3 parts of a mold melamine resin (trade name) were mixed and stirred to obtain a first aqueous base paint 3.
[0149]
[Table 2]

[0150]
The first aqueous base paint obtained in the examples shows excellent values in water resistance and chipping resistance as compared with those obtained in the comparative examples. Not at all. In addition, the adhesiveness to the polypropylene member is good, and no abnormalities such as yellowing are observed by suppressing the content of the volatile basic substance, and the obtained multilayer coating film has excellent finished appearance. Met.
[0151]
【The invention's effect】
The method for forming a multilayer coating film of the present invention comprises the steps of forming a multilayer coating film on a substrate using a first aqueous base coating material, a second aqueous base coating material and a clear coating material, and simultaneously heating and curing. It is a method of forming a layer coating film, and in particular, by using an aqueous coating material having a small amount of residual basic substance, yellowing of the coating film can be suppressed even in a 3 wet 1 bake coating system, and water resistance, chipping resistance, A coating film having low-temperature curability, adhesion to a polypropylene member, and excellent appearance could be formed.
[0152]
That is, the method for forming a multilayer coating film of the present invention can be suitably used in a three-wet coating system aiming at shortening the coating process, reducing costs and reducing environmental load, particularly for coating a vehicle body such as an automobile body. Since the curability is high, it is possible to provide an aqueous base coating composition based on integral coating with a plastic member.

Claims (2)

A step (1) of applying a first aqueous base paint on an object to be coated to form a first base coat, and applying a second aqueous base paint on the first base coat to form a second base coat; A step (2) of forming a film, and further a step (3) of applying a clear paint thereon to form a clear coating, and the first base coating, the second base coating and the clear A method for forming a multilayer coating film comprising a step (4) of simultaneously heating and curing the coating film to obtain a multilayer coating film,
The first aqueous base paint contains an acrylic emulsion resin, an oxazoline group-containing compound, a dispersant pigment dispersion paste, and a thickener, and is formed by steps (1), (2) and (3). The method for forming a multilayer coating film, wherein the coated film has a total amount of volatile basic substances in a unit area of 1 mm ² of 7 × 10 ⁻⁶ mmol or less before performing the step (4). An aqueous base paint used in the method for forming a multilayer coating film according to claim 1, wherein the first aqueous base paint contains an acrylic emulsion resin, an oxazoline group-containing compound, a dispersant pigment dispersion paste, and a thickener. An aqueous base coating composition, characterized in that: