賈修偉,楚紅英,劉治國(guó)
(1.河南大學(xué)藥學(xué)院,河南省高校無(wú)機(jī)離子交換樹(shù)脂工程中心,河南 開(kāi)封475004;2.黃河水利職業(yè)技術(shù)學(xué)院環(huán)境與化學(xué)工程系,河南 開(kāi)封475004;3.西華大學(xué)物理與化學(xué)學(xué)院,四川 成都610039)
環(huán)氧樹(shù)脂在灌封料、層壓復(fù)合材料、絕緣涂層、敷銅板、模塑、灌封材料等領(lǐng)域的應(yīng)用日趨廣泛,對(duì)其阻燃性能的要求亦逐步提高。采用添加型阻燃劑的方式已無(wú)法滿足如微電子、交通、涂料等領(lǐng)域?qū)Νh(huán)氧樹(shù)脂阻燃性能近乎苛刻的要求[1-3],迫使人們不斷探索提高其阻燃性能的新途徑[4-5]。添加型阻燃劑存在噴霜、阻燃性能難以持久等不足,阻燃添加劑往往還起增塑劑作用,不但降低環(huán)氧樹(shù)脂基體的玻璃化轉(zhuǎn)變溫度(Tg),也對(duì)其力學(xué)性能產(chǎn)生負(fù)面影響[6-9]。含鹵和含磷本質(zhì)阻燃環(huán)氧樹(shù)脂不但可以克服添加型阻燃體系的負(fù)面影響,而且能夠獲得持久優(yōu)異的本質(zhì)阻燃性能并基本保持通用環(huán)氧樹(shù)脂的性能;但是,前者燃燒時(shí)易造成二次污染,既損害人們的身心健康,亦腐蝕設(shè)備關(guān)鍵零部件,甚至損壞整臺(tái)設(shè)備等[10];后者磷含量較低且不易控制,阻燃性能有時(shí)并不盡人意,磷元素的加入對(duì)樹(shù)脂的熱穩(wěn)定性能(如Tg下降)有一定影響,最終制品舍棄到環(huán)境中時(shí)造成水的富養(yǎng)化而產(chǎn)生環(huán)保問(wèn)題[11]。因此,人們把環(huán)氧樹(shù)脂分子中的鹵素和磷元素更換為其他環(huán)境友好的阻燃元素硅抑或在環(huán)氧樹(shù)脂中引入特殊結(jié)構(gòu),以獲取優(yōu)異的阻燃和環(huán)境友好性能并且賦予環(huán)氧樹(shù)脂某些優(yōu)良的物化性能。這屬于對(duì)高性能環(huán)氧樹(shù)脂進(jìn)行阻燃處理的極具前景的新方法。本文綜述了含硅及特殊結(jié)構(gòu)本質(zhì)阻燃環(huán)氧樹(shù)脂的發(fā)展、阻燃方法的優(yōu)劣,最后展望了其發(fā)展趨勢(shì)。
硅元素是環(huán)境友好的高效阻燃元素之一[12-14]。含硅基團(tuán)具有較好憎水性和柔順性,硅的表面自由能較低,環(huán)氧樹(shù)脂分子鏈中引入含硅基團(tuán)能提高樹(shù)脂的阻燃性能、韌性、耐潮濕性、耐腐蝕性能、介電性能、熱穩(wěn)定性能、分子鏈的柔順性和材料的表面性能等[15-17]。
含硅本質(zhì)阻燃環(huán)氧樹(shù)脂通常選用攜帶有反應(yīng)活性基團(tuán)(如環(huán)氧基、羥基)的硅烷、硅氧烷或者含硅固化劑[18-20]等進(jìn)行制備。帶環(huán)氧基的含硅化合物的合成相對(duì)困難復(fù)雜,但是它既能夠直接固化,也可以與其他環(huán)氧單體混合之后再固化。含硅基團(tuán)可以位于環(huán)氧樹(shù)脂分子的主鏈、側(cè)鏈上或者固化網(wǎng)絡(luò)結(jié)構(gòu)之中。由于含硅基團(tuán)較強(qiáng)的吸電子效應(yīng)使環(huán)氧基團(tuán)的電負(fù)性降低,含硅環(huán)氧樹(shù)脂的反應(yīng)活性得以提高,易于與固化劑反應(yīng)[21];如果含硅基團(tuán)的體積龐大并且吸電子效應(yīng)較小或者與環(huán)氧基相距較遠(yuǎn),這種反應(yīng)活性的提高并不明顯或者不存在。一種新穎的合成方法是把納米級(jí)POSS籠形結(jié)構(gòu)鍵入環(huán)氧單體分子結(jié)構(gòu)之中,固化后得到納米增強(qiáng)的含硅本質(zhì)阻燃材料,這屬于一種原位生成的納米復(fù)合材料。該納米增強(qiáng)復(fù)合材料與層狀硅酸鹽增強(qiáng)的納米復(fù)合材料有所不同,其分散更均勻、主客體之間存在化學(xué)鍵合、客體無(wú)需預(yù)先進(jìn)行有機(jī)改性;并且POSS分子上的有機(jī)基團(tuán)易調(diào)控性、使用的便利性,為其制備提供了有利條件[22-23]。但是硅倍半氧烷單體的合成條件復(fù)雜苛刻、收率偏低、價(jià)格偏高、純度較難達(dá)到所需標(biāo)準(zhǔn)、向環(huán)氧單體分子引入過(guò)程復(fù)雜[24],所以該方法需要更多的基礎(chǔ)研究和實(shí)際應(yīng)用探索,以便較快由實(shí)驗(yàn)室轉(zhuǎn)入應(yīng)用領(lǐng)域。
與相應(yīng)常規(guī)環(huán)氧樹(shù)脂相比,含硅環(huán)氧樹(shù)脂的Tg一般呈下降趨勢(shì)[25-29],主要源于以下幾方面:(1)環(huán)氧當(dāng)量較高,固化后的交聯(lián)密度較低;(2)Si—O—Ph或者Si—O—C鏈段柔韌性較好,引入環(huán)氧單體分子主鏈之后導(dǎo)致Tg降低[30];(3)含硅基團(tuán)位于環(huán)氧樹(shù)脂的側(cè)鏈時(shí),如果該基團(tuán)體積較大,不但增大固化物的自由基體積,而且降低交聯(lián)密度,Tg下降幅度更大。顯然,環(huán)氧樹(shù)脂中引入硅烷基團(tuán),有時(shí)對(duì)熱穩(wěn)定性的改善并不顯著,因此引入的含硅基團(tuán)需要進(jìn)行精心選擇才能達(dá)到預(yù)期目的[31-34],譬如采用含有剛性基團(tuán)的固化劑可以提高Tg,體積龐大的硅烷基團(tuán)(如二苯基硅氧烷:—O—SiPh2—)引入環(huán)氧單體分子主鏈中同樣有助于提高環(huán)氧樹(shù)脂的熱穩(wěn)定性能。
雖然含硅環(huán)氧樹(shù)脂的熱穩(wěn)定性有時(shí)提高幅度不太理想,但是高溫度范圍內(nèi)(>600℃)熱穩(wěn)定性的較大提高卻是普遍現(xiàn)象。原因在于低表面能的硅遷移到炭層表面形成結(jié)構(gòu)連續(xù)、抗氧化、熱穩(wěn)定性能較強(qiáng)的硅酸鹽保護(hù)層,對(duì)焦炭層及其下層的基材起到良好的保護(hù)作用,使之免遭破壞,炭層的熱穩(wěn)定性得到強(qiáng)化,高溫區(qū)域抗熱氧化性能明顯提高。非含硅環(huán)氧樹(shù)脂的炭層不存在此類熱穩(wěn)定性[35-36]。這種含硅酸鹽的炭層通過(guò)隔熱、斷絕氧的供應(yīng)和阻止高聚物降解生成的可燃性揮發(fā)物的逸出、防止熔體滴落、促使下層基材停止燃燒等途徑起到極佳的阻燃作用,并有一定抑煙作用。含硅環(huán)氧樹(shù)脂的這些優(yōu)勢(shì)類似于膨脹型阻燃體系的阻燃功能:既提升阻燃性能,又使材料受熱和燃燒過(guò)程中生成的煙霧和腐蝕性氣體大大降低。硅元素的加入亦能使基材高溫下的焦炭生成量增加,焦炭生成量隨硅量的提高而提高,但是有些體系無(wú)此現(xiàn)象。
把磷和硅一并鍵入環(huán)氧樹(shù)脂分子結(jié)構(gòu)中,高溫下,磷促進(jìn)富磷焦炭層的生成,硅既益于炭層的生成也改善炭層的熱穩(wěn)定性,因此兩者的協(xié)同阻燃效應(yīng)可使環(huán)氧樹(shù)脂獲得更為理想的阻燃性能和熱穩(wěn)定性能[11],并且能夠獲得較好的力學(xué)性能[37-38]。
硅/磷協(xié)同本質(zhì)阻燃環(huán)氧樹(shù)脂體系中,Si—P的鍵能較低,受熱過(guò)程中首先裂解,導(dǎo)致固化物熱穩(wěn)定性有所降低(初始降解溫度下降),但是熱分解溫度仍然高于300℃,不低于主鏈中僅含磷元素的環(huán)氧樹(shù)脂;這對(duì)富磷焦炭層的生成產(chǎn)生促進(jìn)作用,賦予焦炭層較好的阻隔性能[39-41]。硅元素提高焦炭層的熱穩(wěn)定性和抗高溫?zé)嵫趸阅?,減少高溫區(qū)間內(nèi)熱氧化反應(yīng)造成的質(zhì)量損失,亦即對(duì)該富磷炭層具有保護(hù)作用[42-43]。所以,磷/硅協(xié)同作用可以提高基體的焦炭生成量。炭生成量與相應(yīng)聚合物(非鹵代聚合物)的阻燃性能呈線性關(guān)系[44],而環(huán)氧樹(shù)脂的成炭性能顯著影響其極限氧指數(shù)[45-46],因此,焦炭產(chǎn)量的提高、富磷炭層的易于生成和富磷焦炭層穩(wěn)定性的改善以及樹(shù)脂極限氧指數(shù)的升高意味著體系阻燃性能明顯提高??傊?,硅/磷協(xié)同體系是通過(guò)提高聚合物的炭生成量及其焦炭層的熱穩(wěn)定性來(lái)增加其阻燃性能[47],但是有些體系[48-49]未發(fā)現(xiàn)硅/磷的協(xié)同阻燃效應(yīng),原因有待進(jìn)一步研究。
含硅環(huán)氧單體選用含氮(如含三聚氰胺基)固化劑可以獲得制備硅/氮協(xié)同本質(zhì)阻燃環(huán)氧樹(shù)脂。該樹(shù)脂阻燃性能的改善主要通過(guò)下述機(jī)理達(dá)到阻燃目的[33,50-53]:三聚氰胺基團(tuán)通過(guò)脫氨反應(yīng)放出非燃性氨氣,并形成熱穩(wěn)定和抗熱氧化性能較好的含密胺/蜜勒胺/硅的炭層,利用氣相和凝聚相機(jī)理改善阻燃性能,氣相機(jī)理貢獻(xiàn)較大。相對(duì)于硅/磷協(xié)同體系而言,該體系的初始降解溫度未下降,對(duì)環(huán)境更為友好,但是焦炭生成量的提高幅度不明顯,體系極限氧指數(shù)的漲幅也不及磷/硅協(xié)同本質(zhì)阻燃固化體系,因此,硅/氮協(xié)同體系 阻 燃 效 果 稍 弱 于 硅/磷 協(xié) 同 體 系[25,54-55]。 最 近,Narula等[56]報(bào)道一種磷/硅/氮協(xié)同阻燃環(huán)氧樹(shù)脂體系,阻燃效果良好,將是潛在的優(yōu)良電子電器封裝材料。
采用添加型阻燃劑或者反應(yīng)型阻燃劑實(shí)現(xiàn)有機(jī)高分子材料的阻燃是不得已而為之,大部分情況下是以犧牲被阻燃材料的其他性能如力學(xué)性能為代價(jià)。所以,阻燃技術(shù)的發(fā)展方向是不添加阻燃劑或不引入阻燃元素即可使有機(jī)高分子材料獲取理想阻燃性能,這是人們給予有機(jī)高分子材料阻燃性能、減少火災(zāi)發(fā)生的理想方式,符合“綠色阻燃材料”的要求。特殊結(jié)構(gòu)本質(zhì)阻燃環(huán)氧樹(shù)脂是指通過(guò)調(diào)控環(huán)氧樹(shù)脂的分子結(jié)構(gòu)及組成、固化網(wǎng)絡(luò)結(jié)構(gòu)(包括交聯(lián)密度)而不使用阻燃劑和阻燃元素即獲得阻燃性能和熱穩(wěn)定性能的特殊阻燃環(huán)氧樹(shù)脂。
最初人們?cè)O(shè)想單純依賴提高環(huán)氧樹(shù)脂基體的熱穩(wěn)定性就可以改善阻燃性能,一般是利用多官能團(tuán)環(huán)氧單體增加環(huán)氧樹(shù)脂固化體系的交聯(lián)密度,同時(shí)在固化網(wǎng)絡(luò)中導(dǎo)入大體積的基團(tuán)以減少自由基體積,限制固化網(wǎng)絡(luò)結(jié)構(gòu)中分子鏈段高溫下的運(yùn)動(dòng),但是并未獲得預(yù)期結(jié)果[57-59],只有少數(shù)體系才能獲得理想阻燃性能,大部分情況下只是樹(shù)脂基體熱性能的提高,阻燃性能未能如愿[46,57]。其原因可能是固化體系的熱穩(wěn)定性能并非是影響阻燃性能的主要因素,較高的熱穩(wěn)定性只能提高樹(shù)脂基體的熱降解溫度,進(jìn)而延緩熱降解速度;換言之,較高的熱穩(wěn)定性只是降低了基體向燃燒區(qū)域提供可燃性氣態(tài)物質(zhì)的速度,并不能有效遏制外界氧的傳輸及熱量向樹(shù)脂基體的反饋;而欲有效控制樹(shù)脂引燃和燃燒過(guò)程中的傳熱傳質(zhì)過(guò)程,受熱和燃燒過(guò)程中固化體系應(yīng)當(dāng)擁有形成穩(wěn)定膨脹焦炭層的能力;高交聯(lián)密度能夠提高樹(shù)脂的熱穩(wěn)定性,但是焦炭層的穩(wěn)定性變差,導(dǎo)致樹(shù)脂阻燃性能的提升空間受限,而且過(guò)高的交聯(lián)密度必然會(huì)影響樹(shù)脂基體的力學(xué)性能甚至正常使用,因此,不能為提高熱穩(wěn)定性而無(wú)限制地提高交聯(lián)密度,固化體系必須保持足夠韌性,基體的交聯(lián)密度必須適當(dāng)。
進(jìn)一步的研究顯示,樹(shù)脂結(jié)構(gòu)中引入芳香基團(tuán)能夠獲得優(yōu)良阻燃性能,比如,阻燃性能極佳的酚醛樹(shù)脂和甲酚酚醛樹(shù)脂,2種樹(shù)脂固化環(huán)氧樹(shù)脂時(shí)能夠使后者獲得較好阻燃性能[60],原因是苯酚結(jié)構(gòu)具有優(yōu)良成炭能力;若環(huán)氧樹(shù)脂基體分子結(jié)構(gòu)中同時(shí)再引入縮聚芳香基團(tuán)[46]包括(聯(lián)苯、萘環(huán)、蒽環(huán)、蒽酮衍生物和氟)的衍生物、支化結(jié)構(gòu)基團(tuán)如叔丁基[57]、擁有Diels-Alder反應(yīng)能力的雙鍵如單、雙或三羥基苯乙烯基吡啶[59]等,不但強(qiáng)化成炭性能,而且固化體系的抗裂解能力也同時(shí)得到改善,這意味著固化體系受熱燃燒時(shí)形成的膨脹炭層獲得優(yōu)異熱穩(wěn)定性能。所以,改性后固化體系在不添加阻燃劑的情況下?lián)碛辛钊藵M意的阻燃性能(一般可達(dá)到UL 94V-1級(jí))。另外,該類樹(shù)脂固化體系的交聯(lián)密度相對(duì)較低。
Masatoshi等[61-62]采用這一方法制得特殊結(jié)構(gòu)環(huán)氧樹(shù)脂體系,不使用阻燃添加劑時(shí),樹(shù)脂的阻燃性能達(dá)到UL 94V-0級(jí),極限氧指數(shù)大于40%。他們主要是利用添加特殊結(jié)構(gòu)的酚醛樹(shù)脂衍生物來(lái)調(diào)控環(huán)氧樹(shù)脂固化體系的局部交聯(lián)密度,使體系獲得理想彈性性能,以使固化體系燃燒初期形成具有良好抗熱裂解性能的泡沫狀表面層(泡沫層主要由樹(shù)脂、焦炭、樹(shù)脂降解產(chǎn)生的揮發(fā)性物質(zhì)等組成),該表面層覆蓋在基材表面,能夠很好地阻止熱降解生成的可燃性揮發(fā)物質(zhì)和燃燒釋放的熱量等的傳輸和擴(kuò)散。交聯(lián)密度過(guò)高導(dǎo)致固化體系硬度太大,抗熱裂解性能較低,難于形成穩(wěn)定泡沫狀表面層,阻燃性能相應(yīng)較差;交聯(lián)密度過(guò)低,則固化體系熱穩(wěn)定性能下降。
在這些酚醛樹(shù)脂分子結(jié)構(gòu)中導(dǎo)入阻燃元素或者阻燃基團(tuán)可以更好地發(fā)揮阻燃效能,進(jìn)一步提升固化體系的阻燃性能。譬如苯胍胺(benzoguanamine)改性的雙亞苯基酚醛樹(shù)脂[63],該改性樹(shù)脂受熱或者燃燒時(shí),苯胍胺單元釋放出非可燃性含氮化合物,增強(qiáng)自身與環(huán)氧樹(shù)脂基體的反應(yīng)能力,同時(shí)稀釋燃燒區(qū)域的氧和可燃性氣體物質(zhì);亞苯基鏈段則改善樹(shù)脂的耐水性、熱穩(wěn)定性,同時(shí)亦有助于阻燃性能的提高,在確保耐水性和力學(xué)性能的前提下,提高Tg。環(huán)氧單體中引入酰亞胺基、異氰酸酯基同樣能獲得較好的阻燃效果[63-64]。徐偉箭等[65]在環(huán)氧化合物分子主鏈中引入芳香族雙席夫堿結(jié)構(gòu),該結(jié)構(gòu)具有剛性且含有大量芳香環(huán)和較高氮元素,一方面能夠降低樹(shù)脂分子鏈段高溫范圍內(nèi)的運(yùn)動(dòng)能力,提高其熱穩(wěn)定性,另一方面能夠促進(jìn)焦炭層的生成和提高成炭率,樹(shù)脂阻燃性能達(dá)到UL 94V-0級(jí),800℃成炭率為43.55%。實(shí)際上這屬于氮元素/芳香環(huán)結(jié)構(gòu)共同給予環(huán)氧樹(shù)脂本質(zhì)阻燃性的方法,是含氮本質(zhì)阻燃方法的改進(jìn),改進(jìn)的出發(fā)點(diǎn)是基于如下考慮:?jiǎn)渭兒h(huán)氧樹(shù)脂阻燃性能的發(fā)揮主要通過(guò)氣相機(jī)理,欲提高阻燃性能需提高氮用量,但是樹(shù)脂基體的抗熱降解性能隨之降低,阻燃性能相應(yīng)下降,有些含氮基團(tuán)還可能導(dǎo)致樹(shù)脂基體耐潮濕性能的惡化[66],簡(jiǎn)言之,單靠氮元素?zé)o法實(shí)現(xiàn)預(yù)期的阻燃目標(biāo);而氮元素/芳香環(huán)兩者共同作用的結(jié)果是能夠使樹(shù)脂基體通過(guò)氣相和凝聚相產(chǎn)生阻燃作用,阻燃性能明顯升高,同時(shí)亦能提高樹(shù)脂基體的耐濕性。環(huán)氧化合物與其他熱固性化合物共聚也是極具前途的阻燃途徑,采用該方法得到的改性環(huán)氧樹(shù)脂的阻燃性能和力學(xué)性能均較理想[67]。
研究特殊結(jié)構(gòu)本質(zhì)阻燃環(huán)氧樹(shù)脂的目的主要為滿足電子電器(尤其是微電子)行業(yè)所需高物化性能高熱穩(wěn)定性環(huán)氧樹(shù)脂綠色阻燃的苛刻要求。該類特殊結(jié)構(gòu)環(huán)氧樹(shù)脂除擁有優(yōu)異的阻燃性能之外,尚擁有可模塑、易封裝等優(yōu)異性能,某些諸如耐熾熱、耐潮濕以及高溫存放等性能優(yōu)于目前添加型阻燃劑阻燃的高性能環(huán)氧樹(shù)脂體系[66,68],應(yīng)用于印刷線路板和集成電路模塑材料時(shí),充分滿足無(wú)鉛焊接的高溫要求,操作簡(jiǎn)單,符合綠色化學(xué)大趨勢(shì)。日本NEC公司等已將這種新型印刷線路板應(yīng)用于制造環(huán)境友好型的電子電器。
采用賦予環(huán)氧樹(shù)脂特殊結(jié)構(gòu)提高其阻燃性能的方法一般需要精心選擇原料(包括無(wú)機(jī)填料)、控制產(chǎn)物的結(jié)構(gòu)和組成[67-71],否則,或者阻燃效率達(dá)不到預(yù)期目標(biāo)、需添加阻燃型樹(shù)脂,或者力學(xué)性能受到影響。另外,此類本質(zhì)阻燃樹(shù)脂研究剛剛起步,許多未知領(lǐng)域比如結(jié)構(gòu)(包括組成及數(shù)量等)與阻燃性能以及物化性能等的關(guān)系、阻燃機(jī)理等需要研究。
硅本質(zhì)阻燃環(huán)氧樹(shù)脂和特殊結(jié)構(gòu)本質(zhì)阻燃環(huán)氧樹(shù)脂能夠克服添加型阻燃環(huán)氧樹(shù)脂和含磷或含鹵本質(zhì)阻燃環(huán)氧樹(shù)脂的缺點(diǎn),具有阻燃效率高、無(wú)遷移之虞、基材的力學(xué)性能所受影響較小、環(huán)境友好等優(yōu)勢(shì)。通過(guò)選擇阻燃基團(tuán)尚可以賦予環(huán)氧樹(shù)脂特殊物化性能。利用阻燃協(xié)同效應(yīng),在保持阻燃級(jí)別不變的前提下,可以降低阻燃元素的用量。2種本質(zhì)阻燃環(huán)氧樹(shù)脂的研究和開(kāi)發(fā),使環(huán)氧樹(shù)脂體系獲得高效低毒、環(huán)境友好等性能逐漸明朗化,而這些本質(zhì)阻燃體系的實(shí)際應(yīng)用最終將給人們帶來(lái)極大益處。
環(huán)氧單體、固化劑、固化條件等對(duì)固化網(wǎng)絡(luò)結(jié)構(gòu)(如交聯(lián)密度、自由體積)的影響較大,不但對(duì)最終制品的阻燃性能和熱性能起決定性作用,亦明顯影響力學(xué)性能。在環(huán)氧單體分子、固化劑分子中引入阻燃基團(tuán)或者阻燃元素需要考慮引入后分子反應(yīng)性能的變化及變化趨勢(shì)、對(duì)固化反應(yīng)和固化樹(shù)脂力學(xué)性能的影響等,阻燃機(jī)理、結(jié)構(gòu)組成與相應(yīng)體系的物化性能及阻燃性能的關(guān)系等也需要深層次探索,結(jié)構(gòu)、性能和加工三者間的關(guān)系同樣不容忽視。合成和加工工藝復(fù)雜、制備成本過(guò)高、難于大規(guī)模工業(yè)化生產(chǎn)和應(yīng)用,絕大部分含硅和特殊結(jié)構(gòu)本質(zhì)阻燃環(huán)氧樹(shù)脂目前尚處于實(shí)驗(yàn)室階段,因此簡(jiǎn)化合成工藝、降低生產(chǎn)成本是今后的一個(gè)發(fā)展熱點(diǎn)。本質(zhì)阻燃環(huán)氧樹(shù)脂的研究重點(diǎn)是開(kāi)發(fā)出兼顧綠色化學(xué)發(fā)展趨勢(shì)、低毒抑煙、生態(tài)友好、物化性能優(yōu)良、阻燃效率理想等綜合性能的本質(zhì)阻燃體系。
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