彭叔森，曾志翔，韓 金，陳建敏，烏學(xué)東

(中國科學(xué)院寧波材料技術(shù)與工程研究所 中國科學(xué)院海洋新材料與應(yīng)用技術(shù)重點實驗室浙江省海洋材料與防護技術(shù)重點實驗室， 浙江 寧波 315201)

1 前 言

溶膠-凝膠技術(shù)是一種由金屬有機化合物、金屬無機化合物或它們的混合物經(jīng)水解縮合過程，逐漸凝膠化及進行相應(yīng)后處理，而獲得氧化物或其它化合物的工藝?，F(xiàn)代溶膠-凝膠技術(shù)的歷史可以追溯到1846年，法國科學(xué)家Ebelmen發(fā)現(xiàn)SiCl4與乙醇混合后在濕空氣中會水解形成凝膠[1]。經(jīng)過近兩個世紀的發(fā)展，溶膠-凝膠技術(shù)已被廣泛的用于制備體材料、纖維材料、薄膜材料及納米粉體等[2]。早期用于金屬表面上的溶膠-凝膠涂層從其作用上可以分為抗氧化涂層、耐蝕涂層、附著促進層、耐刮擦涂層及絕緣涂。Guglielmi[3]于1997年撰文總結(jié)了用于金屬表面的溶膠-凝膠涂層的工作，從該文獻可知涂層材料多為無機材料。無機溶膠-凝膠涂層的優(yōu)點是具有高的熱穩(wěn)定性及在金屬上有好的附著性能,其優(yōu)異的附著性能也使得溶膠-凝膠涂層作為一種取代鉻鈍化和磷化膜的技術(shù)而被廣泛研究。不過，盡管無機溶膠-凝膠涂層能夠提供很好的腐蝕保護效果，然而其應(yīng)用受限于：①無機氧化物涂層的高內(nèi)應(yīng)力、高脆性使得難以獲得無裂痕的厚膜(>1 μm)；②需要相對的高溫(400～800 ℃)才能獲得較好的性能[4]。

有機-無機雜化材料兼具了有機材料和無機材料的特性，而成為新材料的研發(fā)熱點[6]。通過改變無機、有機組分可以連續(xù)的改變材料的性質(zhì)而獲得所需的性能。無機成分可以獲得好的耐刮擦、耐久性能及在金屬上優(yōu)異的附著性能；而有機組份提高材料的致密性、韌性以及帶官能基團等。溶膠-凝膠技術(shù)是一種制備有機-無機雜化材料的優(yōu)異的手段，關(guān)于這方面的工作可以參閱Wilkes等人的綜述文章[7-8]。為了解決純無機溶膠-凝膠涂層的高脆性、降低其成膜溫度，有機成分被添加到體系中以來獲得優(yōu)異的性能。目前主要有3種途徑獲得有機-無機雜化溶膠-凝膠涂層[9]：簡單混合有機組分和無機溶膠-凝膠，兩成分間無化學(xué)鍵作用(圖1a);利用有機聚合物/低聚物中的官能團與無機前驅(qū)體反應(yīng)，兩者間通過化學(xué)鍵連結(jié)(圖1b);使用烷基烷氧基硅烷作為溶膠-凝膠前軀體或與金屬醇鹽、Si(OR)4一起作為起溶膠-凝膠的前軀體，所得的主鏈為無機Si-O-Si或Si-O-M(M為Zr或Ti)結(jié)構(gòu)，有機成份為側(cè)基懸掛在硅原子上(圖1c)。在這3種途徑中，以有機硅為前驅(qū)體的方法使用最為廣泛。這是因為烷基烷氧基硅烷的有機活性基團多種多樣、Si-OR水解反應(yīng)溫和、多數(shù)烷基烷氧基硅烷為商業(yè)化、批量生化產(chǎn)的產(chǎn)品。近年來，有機硅溶膠-凝膠防腐蝕涂層已成為防腐蝕材料領(lǐng)域的研究熱點。本文從有機硅溶膠的制備的基本反應(yīng)、硅烷在金屬表面上的成膜機理以及在不同金屬上的應(yīng)用等方面進行介紹。

2 有機硅溶膠-凝膠涂層

圖1 溶膠-凝膠技術(shù)制備有機-無機雜化材料的結(jié)構(gòu)示意圖[5] Fig.1 Structural models of different classes of organic-inorganic hybrid materials formed by the sol-gel process[5]

有機硅溶膠-凝膠涂層是指以烷基烷氧基硅烷為前驅(qū)體通過溶膠-凝膠技術(shù)制備的涂層。烷基烷氧基硅烷是一類天然的有機-無機雜化分子：一端是可水解的Si-OR基團；另一端是各種有機基團如氨基、環(huán)氧、巰基、雙鍵、異氰基官能等。這類有機硅化合物作為偶聯(lián)劑而廣泛應(yīng)用于有機聚合物復(fù)合材料的制備、高分子化合物改性以及有機-無機雜化材料的合成等領(lǐng)域。圖2給出了一些常見的烷基烷氧基硅烷和烷氧基硅烷的分子結(jié)構(gòu)示意圖。有機硅分子在溶膠-凝膠過程中主要涉及Si-OR的水解及Si-OH的縮合反應(yīng)，下面專門進行介紹。

圖2 一些常見烷基烷氧基硅烷和烷氧基硅烷的分子結(jié)構(gòu)Fig.2 Molecular structure of alkylalkoxysilanes and alkoxysilanes

2.1 烷氧基硅烷和烷基烷氧基硅烷的反應(yīng)

烷氧基硅烷在含水溶液中最為重要的反應(yīng)是水解和縮合反應(yīng)[10]。這兩個反應(yīng)會影響硅烷分子的溶解性、硅烷溶液的穩(wěn)定性[11]、反應(yīng)活性以及所得產(chǎn)物的結(jié)構(gòu)，因此已有大量關(guān)于其反應(yīng)機理的研究。第一篇相關(guān)的研究報道見于1844年，Ebelmen研究了四乙氧基硅烷(TEOS)的水解和縮合反應(yīng)[12]，并發(fā)現(xiàn)縮聚反應(yīng)伴隨水解反應(yīng)的進行而進行。Konrad[13]系統(tǒng)的研究了水在四甲氧基硅烷(TMOS)水解中起到的作用，其結(jié)果顯示當(dāng)水量少于可水解基團的時，產(chǎn)物更偏向形成線性分子，而當(dāng)水過量時產(chǎn)物偏向形成三維網(wǎng)狀結(jié)構(gòu)。Schmidt[14]撰文這方面的工作進行了詳細的綜述。

對烷基烷氧基硅烷的水解-縮合的研究也有不少的報道，如已有報道研究了甲基三乙氧基(MTES)[15]，乙烯基三甲氧基硅烷(VTMS)和乙烯基三乙氧基硅烷(VTES)[15]，3-縮水甘油氧基丙基三甲氧基硅烷(γ-GPS)[16]，γ-GPS和胺丙基三乙基硅烷(γ-APS)混合體系[17]，γ-UPS[17]，甲基丙烯酰氧基丙基三甲氧基硅烷(γ-MPS)[18]，苯基三乙氧基硅烷(PHTS)[19]，胺丙基三乙氧基硅烷(γ-APS)，1,2-二(三乙氧基硅基)乙烷(BTSE)[20]，1,2-二(3-三甲氧基甲硅烷基丙基)胺(BTSA)和乙烯基三乙酰氧基(VTSA)的混合體系[21]，VTSA[22]，丙基三甲氧基硅烷(PTS)[23]的水解-縮合反應(yīng)。研究手段多為IR，Raman，1H-NMR，29Si-NMR等?？偟膩碚f，影響烷基烷氧基硅烷的反應(yīng)活性的因素[24]主要有烷氧基的體積效應(yīng)[25]、連接在硅原子上的烷基、有機溶劑、溶液濃度、pH值及溫度。

2.2 硅烷在金屬表面上的成鍵機理

硅烷在金屬表面的成鍵機理一般認為是硅醇能和金屬表面上的羥基通過脫水反應(yīng)形成Si-O-Me鍵，其反應(yīng)如圖3所示。該機理由Plueddemann[26]提出，也為已有的一些研究所證實。如Getting等[27]利用SIMS和XPS研究了γ-GPS在鋼材上形成薄膜。SIMS結(jié)果顯示在硅烷處理過的鋼材上得到Fe-O-Si+峰，因此他們稱硅烷在鋼上形成Si-O-Fe鍵而結(jié)合到鐵基體上。Watts等[28]用SIMS和XPS研究了γ-GPS在鐵基體上形成的硅烷薄膜，同樣發(fā)現(xiàn)Fe-O-Si+的存在。在γ-GPS處理過的鋁合金表面上，Leung等[29]通過XPS分析得到了Si-O-Al的信號。Teo等[30]用SIMS研究了經(jīng)BTSE處理的鋁表面發(fā)現(xiàn)了Al-O-Si+的峰。Fang等[31]用SIMS和XPS研究了γ-GPS處理過的鋁合金表面，獲得了Al-O-Si+的信息。從文獻報道來看，關(guān)于硅烷在金屬表面上的成鍵機理還需進一步的研究，還需要更多的數(shù)據(jù)支持。

圖3 硅羥基和金屬表面上的羥基的反應(yīng)示意圖Fig.3 Schematic representation of condensation of silanol with hydroxyl groups on the metal surface

2.3 有機硅溶膠-凝膠涂層在不同金屬上的應(yīng)用

Schmidt等[32]在1994年撰文介紹了有機-無機雜化溶膠-凝膠涂層在不同場合下的應(yīng)用，其中提到有機硅溶膠-凝膠涂層用作金屬防腐涂層。1997年，Pilz等[33]采用多種硅烷制備了用于青銅戶外保護的溶膠-凝膠涂層。1998年，Langenfeld等[34]研究了基于γ-GPS的溶膠-凝膠涂層對鎂、鋁、鋅、黃銅的保護。1999年，Metroke等[35]以γ-GPS和TEOS為前驅(qū)體制備了雜化溶膠-凝膠涂層，并用鹽霧測試評估了該涂層對鋁合金的保護效果。進入2000年后，有機硅溶膠-凝膠涂層也得到了飛速發(fā)展。表1給出了常用硅烷及其它前驅(qū)體的縮寫及化學(xué)名稱。

表2總結(jié)了2000年后有關(guān)于有機硅溶膠-凝膠防腐蝕涂層的文獻報道。文獻按照金屬基體進行了分類并列出了所用的前驅(qū)體(縮寫及化學(xué)名稱見表1)及添加物。從金屬基體看有機硅溶膠-凝膠防腐蝕涂層主要應(yīng)用于鋁基、鐵基、銅基及鎂基材料。其中在鋁基材料研究最多，在純鋁及各種型號的鋁合金上都有相關(guān)的研究。鐵基材料除純鐵外，還涉及碳鋼、不銹鋼及鍍鋅鐵。在銅基材料上主要為紫銅。隨著近年來鎂合金的應(yīng)用推廣，利用有機硅溶膠-凝膠涂層對鎂合金的防護研究也日趨增多。研究報道的多少在一定程度上也反映了有機硅溶膠-凝膠涂層在該金屬基材料上應(yīng)用情況。從應(yīng)用上看，在鋁合金最為成功，國外已有取代鉻鈍化的應(yīng)用，在銅上的效果較差。這是因為涂層的保護性能取決于涂層在金屬表面上的附著性能，而有機硅溶膠-凝膠涂層與金屬間的附著取決于是否易形成Si-O-Metal鍵。已有的研究證實Si-O-Al易形成[31]，而Si-O-Cu是難以形成的[36]。

從有機基團的角度可以將有機硅溶膠-凝膠涂層分為以下幾大類：①有機基團為非活性基團的甲基、苯基，如以MTMS[37]，PhTS[38]為前驅(qū)體制備的涂層等；②有機活性基團帶有環(huán)氧基團，烷基烷氧基硅烷多為γ-GPS；③有機活性基團帶有雙鍵，烷基烷氧基硅烷多為γ-MPS或VTMS；④有機活性基團帶有氨基，如以γ-APS為前驅(qū)體[39]。第一類適合作為單獨的保護涂層，如MTES/TEOS的溶膠-凝膠涂層具有優(yōu)異的耐高溫特性[40]。而帶有環(huán)氧、雙鍵、氨基的涂層，能夠和有機涂層通過這些基團形成化學(xué)鍵，因此除作為單獨的保護涂層，還適合作為有機涂層的打底涂層提高有機涂層在金屬上的附著性能。除這些基團，一些含有特殊基團的涂層也被設(shè)計。如Khramov等[41]以PHS和TEOS為前軀體制備了含有膦基的涂層，該涂層可以通過膦基和鎂合金形成化學(xué)鍵；含巰基的涂層被用來獲得在銅上有好的性能的[42-44]基于巰基可以和銅形成Cu-S-C鍵。

表1 常用硅烷及其它前驅(qū)體的縮寫及化學(xué)名稱

為了進一步提高有機硅溶膠-凝膠涂層的防腐蝕性能有機和無機緩蝕劑[45-46]被添加到涂層中。緩蝕劑的濃度在一定的范圍內(nèi)可以有效的增強涂層的性能，但是當(dāng)濃度過高時會破壞溶膠-凝膠涂層的結(jié)構(gòu)而降低性能[47]。一些報道還研究了微米、納米顆粒對涂層性能的影響[48-49]。

2.4 有機硅溶膠-凝膠涂層在海洋防腐蝕的應(yīng)用

盡管目前有關(guān)于有機硅溶膠-凝膠涂層的報道還集中于涂層的設(shè)計及對不同金屬的保護效果的評價，直接針對海洋相關(guān)領(lǐng)域的應(yīng)用研究還非常少見。但有機硅溶膠-凝膠涂層優(yōu)異的耐熱、力學(xué)性能、耐腐蝕、附著性能以及靈活的施工手段使得其在一些特殊的場合有大的應(yīng)用前景。如對耐熱、耐刮擦有一定要求，但一般有機涂層無法滿足要求的場合，或者其它保護如噴涂技術(shù)無法施工的地方。當(dāng)然，有機硅溶膠-凝膠涂層在海洋相關(guān)領(lǐng)域的應(yīng)用還需結(jié)合實際情況具體分析和探討。

表2 不同金屬表面上的有機硅溶膠-凝膠防蝕腐涂層

(接下表) (續(xù)表)

3 結(jié) 語

有機硅溶膠-凝膠涂層因其特殊的性能而廣泛用于金屬材料的防腐蝕，應(yīng)用前景非常廣闊。目前，市面上已有些商業(yè)化的產(chǎn)品。但有機硅溶膠-凝膠涂層也面臨一些亟需解決問題及值得繼續(xù)研究的方向：

(1)多數(shù)有機硅溶膠-凝膠涂層體系含有大量醇，醇的來源有兩類：一是作為溶劑額外添加的，二是前驅(qū)體水解反應(yīng)產(chǎn)生的。大量醇的存在不但會帶來安全隱患，還面臨VOC排放的問題。因此，開發(fā)水基/低VOC有機硅溶膠-凝膠涂層非常有意義。

(2)借鑒有機緩蝕劑在金屬表面上化學(xué)吸附的工作原理，設(shè)計含有特殊官能基團的有機硅溶膠-凝膠涂層，通過有機基團和金屬的相互作用增強涂層在金屬表面(特別是銅)上的附著能力以及具有“自修復(fù)”功能的涂層，擴大有機硅溶膠-凝膠涂層的適用范圍及提升其保護效果。

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