有機(jī)硅溶膠-凝膠防腐蝕涂層研究進(jìn)展

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(diǎn)實(shí)驗(yàn)室浙江省海洋材料與防護(hù)技術(shù)重點(diǎn)實(shí)驗(yàn)室， 浙江 寧波 315201)

1 前 言

溶膠-凝膠技術(shù)是一種由金屬有機(jī)化合物、金屬無機(jī)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(jìn)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(huì)水解形成凝膠[1]。經(jīng)過近兩個(gè)世紀(jì)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(jìn)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(xiàn)可知涂層材料多為無機(jī)材料。無機(jī)溶膠-凝膠涂層的優(yōu)點(diǎn)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(jī)溶膠-凝膠涂層能夠提供很好的腐蝕保護(hù)效果，然而其應(yīng)用受限于：①無機(jī)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(duì)的高溫(400～800 ℃)才能獲得較好的性能[4]。

有機(jī)-無機(jī)雜化材料兼具了有機(jī)材料和無機(jī)材料的特性，而成為新材料的研發(fā)熱點(diǎn)[6]。通過改變無機(jī)、有機(jī)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(jī)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(jī)組份提高材料的致密性、韌性以及帶官能基團(tuán)等。溶膠-凝膠技術(shù)是一種制備有機(jī)-無機(jī)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(jī)溶膠-凝膠涂層的高脆性、降低其成膜溫度，有機(jī)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(jī)-無機(jī)雜化溶膠-凝膠涂層[9]：簡(jiǎn)單混合有機(jī)組分和無機(jī)溶膠-凝膠，兩成分間無化學(xué)鍵作用(圖1a);利用有機(jī)聚合物/低聚物中的官能團(tuán)與無機(jī)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(jī)Si-O-Si或Si-O-M(M為Zr或Ti)結(jié)構(gòu)，有機(jī)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(jī)硅為前驅(qū)體的方法使用最為廣泛。這是因?yàn)橥榛檠趸柰榈挠袡C(jī)活性基團(tuán)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(jī)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(diǎn)。本文從有機(jī)硅溶膠的制備的基本反應(yīng)、硅烷在金屬表面上的成膜機(jī)理以及在不同金屬上的應(yīng)用等方面進(jìn)行介紹。

2 有機(jī)硅溶膠-凝膠涂層

圖1 溶膠-凝膠技術(shù)制備有機(jī)-無機(jī)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(jī)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(jī)-無機(jī)雜化分子：一端是可水解的Si-OR基團(tuán)；另一端是各種有機(jī)基團(tuán)如氨基、環(huán)氧、巰基、雙鍵、異氰基官能等。這類有機(jī)硅化合物作為偶聯(lián)劑而廣泛應(yīng)用于有機(jī)聚合物復(fù)合材料的制備、高分子化合物改性以及有機(jī)-無機(jī)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(jī)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(jìn)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(gè)反應(yīng)會(huì)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(jī)理的研究。第一篇相關(guān)的研究報(bào)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(jìn)行而進(jìn)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(tuán)的時(shí)，產(chǎn)物更偏向形成線性分子，而當(dāng)水過量時(shí)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(jìn)行了詳細(xì)的綜述。

對(duì)烷基烷氧基硅烷的水解-縮合的研究也有不少的報(bào)道，如已有報(bào)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(jī)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(jī)理

硅烷在金屬表面的成鍵機(jī)理一般認(rèn)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(jī)理由Plueddemann[26]提出，也為已有的一些研究所證實(shí)。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(hào)。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(xiàn)報(bào)道來看，關(guān)于硅烷在金屬表面上的成鍵機(jī)理還需進(jìn)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(jī)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(jī)-無機(jī)雜化溶膠-凝膠涂層在不同場(chǎng)合下的應(yīng)用，其中提到有機(jī)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(hù)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(duì)鎂、鋁、鋅、黃銅的保護(hù)。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(cè)試評(píng)估了該涂層對(duì)鋁合金的保護(hù)效果。進(jìn)入2000年后，有機(jī)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(jī)硅溶膠-凝膠防腐蝕涂層的文獻(xiàn)報(bào)道。文獻(xiàn)按照金屬基體進(jìn)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(jī)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(hào)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(jī)硅溶膠-凝膠涂層對(duì)鎂合金的防護(hù)研究也日趨增多。研究報(bào)道的多少在一定程度上也反映了有機(jī)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?yàn)橥繉拥谋Ｗo(hù)性能取決于涂層在金屬表面上的附著性能，而有機(jī)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(shí)Si-O-Al易形成[31]，而Si-O-Cu是難以形成的[36]。

從有機(jī)基團(tuán)的角度可以將有機(jī)硅溶膠-凝膠涂層分為以下幾大類：①有機(jī)基團(tuán)為非活性基團(tuán)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(jī)活性基團(tuán)帶有環(huán)氧基團(tuán)，烷基烷氧基硅烷多為γ-GPS；③有機(jī)活性基團(tuán)帶有雙鍵，烷基烷氧基硅烷多為γ-MPS或VTMS；④有機(jī)活性基團(tuán)帶有氨基，如以γ-APS為前驅(qū)體[39]。第一類適合作為單獨(dú)的保護(hù)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(jī)涂層通過這些基團(tuán)形成化學(xué)鍵，因此除作為單獨(dú)的保護(hù)涂層，還適合作為有機(jī)涂層的打底涂層提高有機(jī)涂層在金屬上的附著性能。除這些基團(tuán)，一些含有特殊基團(tuán)的涂層也被設(shè)計(jì)。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(jìn)一步提高有機(jī)硅溶膠-凝膠涂層的防腐蝕性能有機(jī)和無機(jī)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(qiáng)涂層的性能，但是當(dāng)濃度過高時(shí)會(huì)破壞溶膠-凝膠涂層的結(jié)構(gòu)而降低性能[47]。一些報(bào)道還研究了微米、納米顆粒對(duì)涂層性能的影響[48-49]。

2.4 有機(jī)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(jī)硅溶膠-凝膠涂層的報(bào)道還集中于涂層的設(shè)計(jì)及對(duì)不同金屬的保護(hù)效果的評(píng)價(jià)，直接針對(duì)海洋相關(guān)領(lǐng)域的應(yīng)用研究還非常少見。但有機(jī)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(chǎng)合有大的應(yīng)用前景。如對(duì)耐熱、耐刮擦有一定要求，但一般有機(jī)涂層無法滿足要求的場(chǎng)合，或者其它保護(hù)如噴涂技術(shù)無法施工的地方。當(dāng)然，有機(jī)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(shí)際情況具體分析和探討。

表2 不同金屬表面上的有機(jī)硅溶膠-凝膠防蝕腐涂層

(接下表) (續(xù)表)

3 結(jié) 語

有機(jī)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(jī)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(jī)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(huì)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(jī)硅溶膠-凝膠涂層非常有意義。

(2)借鑒有機(jī)緩蝕劑在金屬表面上化學(xué)吸附的工作原理，設(shè)計(jì)含有特殊官能基團(tuán)的有機(jī)硅溶膠-凝膠涂層，通過有機(jī)基團(tuán)和金屬的相互作用增強(qiáng)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(kuò)大有機(jī)硅溶膠-凝膠涂層的適用范圍及提升其保護(hù)效果。

參考文獻(xiàn) References

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